The higher numbers quoted for the Thief may include the bonus from Backstab, which is completely inappropriate, of course.
Alternatively, they may simply reflect Thief Weapon Talent (+1), Nimble Blade (+1 with CA), and the standard CA bonus (+2). It may also assume that the Thief will start with 20 Dexterity and the Ranger will not.
Again, these assumptions aren't really fair. You can't just say "Rogues always get CA but no one else ever does, so the Rogue always has a +2 advantage." While the Thief (and other Rogues) are the big winners in the CA game, and recent rules changes have made CA almost automatic for them, the fact is that CA is now much easier for any character who makes it a priority. Cunning Stalker, Hidden Sniper and Terrain Advantage (all from HotFK) and Vicious Advantage (PHB3) are available to everyone.
However, you shouldn't just assume the Thief's player is cheating. Dex 20 (+5), Dagger Proficiency (+3), Light Blade Expertise (+1) and CA (+2) is probably the combination that totals +11 for the character in question.
Opps. You're right. I think I accidently gave the Thief +1 for Weapon Talent a level too early.
+10 for the Thief yields 16.175 DPR. Still 4 points higher than the ranged Ranger and that slight gain disappears at level 2.
Cunning Stalker is tougher for ranged Rangers since the foe has to be adjacent. It's good for melee Rangers.
Hidden Sniper is situational. Concealment usually requires a special power or item, so it's really designed for Warlocks.
Terrain Advantage is even more situational.
At first level, the ranged Ranger is probably not going to take any of these compared to Greatbow and if he does, his DPR drops. He might not take any of them until level 6 or higher because there are so many other bread and butter feats that give him more bang for the buck.
If the ranged Ranger could get CA every single round, his DPR would rise to 13.58125. A good reason to take Hidden Sniper if he could only find a way to get concealment (cover is easy, concealment usually takes a power or an item).
We had a Thief in one of our campaigns for 3 levels (that campaign is on hold while we play a different one) and practically the only time that he did not have CA was when he was using an Action Point or an Opportunity Attack. In 3 levels, I think he did not have CA for his main attack once or twice at most.
It's not unfair to give the Thief CA in the calculations because it is a 90% of the time class feature that the ranged Ranger does not have.
CA does have to be considered for a melee Ranger. It still doesn't put him close to the Thief damage-wise until much higher level.