At 1st level, Rangers aren't that special, and Thieves can easily outstrip them. By late heroic, though, Rangers start pulling ahead of anything else for single-target damage because of their ability to double-dip on every single damage booster with Twin Strike and Encounter/Daily powers that are Twin Strike+. Hunter's Quarry is just a sideshow—Twin Strike is the Ranger's striker class feature. At low levels, though, they're still paying down the primary-attribute-modifier investment on their double damage growth rate.
Well, the Rangers do not necessarily pull ahead by late heroric, but they do tend to catch up.
For every +1 damage boost for the Ranger, the Thief can more or less also get it (e.g. magic item, weapon focus feat, etc.).
So with a 65% chance to hit, the Ranger gets 1.3 extra damage (.65*2*1) for every one point extra weapon damage whereas the Thief gets 0.9 extra damage (.9*1, second level Weapon Talent allows the Thief to hit even easier and boosts his 0.85 to 0.9). That's 0.4 points DPR that the Ranger gains per +1 damage per weapon die increase.
With the Thief doing about 5 more points of DPR per round than the Ranger at first level, it takes 12 levels or level 13 to just catch up IF both PCs could manage +1 more weapon damage every single level.
But, they typically do not gain +1 damage on the weapon every single level (although Bracers of Archery help). Plus, that's just to catch up.
Also, the 18 or 20 Dex Thief gets a +1 bonus to damage at level 8, 14, 21 (+2 here), and 28 due to ability score boosts. With Twin Strike, the Ranger does not get these boosts to At Will damage (although it does often work for Encounter or Daily powers).
But the real area where Rangers catch up quickly is with Immediate Reaction and Interrupt Encounter powers outside the Ranger's turn.
That's typically a full extra attack per encounter per Encounter power. So at level 7, the Thief is still doing 3 more points of damage per attack plus 21 more points of Backstab damage or 39 extra points of damage in a 6 round encounter over the Ranger, but the Ranger gets 3 extra attacks per encounter in which to do 39 points of damage. He can often manage that.
It usually take mid-Paragon before the Ranger solidly pulls ahead.
what is this i don't even.
Thieves do not, at level 1, have bonuses to hit that the ranger does not have access to.
Conversely, Rangers have access to Prime Shot which a rogue does not.
No wonder your thief has such high dpr, he's cheating.
Prime Shot?
Are you serious?
I've played with quite a few ranged Rangers and the number of times I've seen Prime Shot used is about once per two encounters. Maybe. The main problem with Prime Shot is that it exposes the ranged Ranger because encounters often start out as NPCs on one side of a map and PCs on the other. The Ranger has to move closer to the NPCs than his allies.
But even IF the Ranger could use Prime Shot every single round, his DPR goes to:
Ranger: +8 to hit, hits 70% of time, DPR 12.845 (12.07125 without Prime Shot)
Thieves can get Combat Advantage nearly every round with their stances and with a team of players who understand how to help the Thief do that, it becomes almost automatic.
How exactly does a low level ranged Ranger get Combat Advantage nearly every round without other PCs giving it to him? Distance Advantage is the only way I can think of and that requires a certain party makeup (i.e. two melee types) and help from his fellow PCs. The other main option is by Hiding, but that too is fairly problematic most rounds. There are very few Ranger powers that give CA to the Ranger and most of them are melee ones. To get CA, the ranged Ranger typically needs help from his fellow PCs. The Thief does not.
The rest of the Thief and Ranger abilities are pretty much a wash with regard to damage. Thieves use Dex for both melee and ranged attacks, so they do not need two ability scores to accomplish that. They can Sneak Attack more than once per round if he can get attacks outside his turn like an Opportunity Attack or a Warlord giving him an extra attack (damage limited to CA still). They can charge for +1 more to hit and melee attack a foe whereas the ranged Ranger cannot. On the other hand, ranged Rangers can use their Action Points do full damage. They get Prime Shot and they do slightly more if an ally gives them an extra attack if they do not have CA. And if they lose CA, they still do good damage.
Where the Ranger catches up is by taking Immediate Reaction or Interrupt Encounter powers instead of powers that do extra damage on an attack.
No contest at low level. The Thief wins the damage contest. Damage-wise, the Ranger doesn't catch up until level 7 when he has 3 Encounter Attack powers (the Daily powers help, but on overall average damage, the Encounter powers are the power curve).
The melee 4E Ranger can get Combat Advantage at least half of the time. But, he does less DPR than the ranged Ranger without the Combat Advantage, so his DPR tends to still be lower unless he gets CA most rounds (i.e. 11.335 DPR with twin strike Bastard Swords and Two Blade Fighting Style, 12.61 DPR with CA).
Note: I haven't done any calculations for either a ranged or melee Ranger from Essentials. It looks like their DPR is quite a bit better than a ranged or melee Ranger from 4E, but not quite enough to catch up to a Thief until the second half of heroic or later.