A bit, but let's consider what we've got here.
Let's start with a Grick. 14 AC, 27 HP, potential to do 13 damage with two +4 to hit attacks. Probably get a surprise round, which grants it advantage to hit once, but otherwise something I'd call a very moderate challenge to a level 2 party.
Grick has damage resistance to nonmagical weapons, which isn't quite as good as twice as many hit points, but it's sort of in that ballpark. And it has a weaker version of multiattack -- instead of just making two attacks, it has to hit with the first one to get a chance at the second.
Move next to a Gnoll Pack Lord, fighting alone. Now we've got 15 AC and 49 HP, with the chance to do 16 damage in two attacks. Tough, and definitely more dangerous, but still the chance of PC death is quite low.
As you say.
Now the Polar/Cave Bear: only 12 AC and 49 HP, but +7 to hit on two attacks for a total of 21 damage each round. It's got a good chance to hit and a good chance to drop a character each round, and 49 HP is easily 2+ rounds for an average party, even focus-fired. Still not a likely TPK but the damage is way up and it's still a CR 2.
Hits harder than the gnoll or grick, but also has lower AC and lower HP than those monsters, so it looks to me to be in the right ballpark. It's also just a beast, so lots of druid/ranger stuff is a possibility, so yeah, maybe on the harder side of CR2, but maybe not.
Ghast - not too tough with 13 AC and 36 HP, but turn defense means clerics no likey and melee will have a bad time with it - save or disadvantage to hit. The main thing is its +5 hit, 10 damage and DC 10 Con save or paralyzed - which means next round you are down (coup-de-grace bite for 19 damage). Sure it takes a couple rounds and a failed save, but the chance of a PC dying goes up.
A lot of things need to go wrong for the hapless character here -- they have to get hit at +5, then fail the Con save, then get hit at +3 with advantage, and then after all that they take 19 (more) damage. Which isn't a good day, but not every day in the front line is.
But what about the Gibbering Mouther? It's a first-round danger: DC 13 Dex save or be blind in an AoE, which gives it advantage to hit you for a whopping 17 damage. That means someone is very likely to go down in one round - which combos in a nasty way with its "DC 10 Wis save or be useless" power that happens EVERY ROUND to EVERYONE. So there's a decent chance you'll helplessly watch your downed buddy bleed out while you hit your friend or stand in a corner, especially since it has 67 HP to chew through. Chance of death goes way up.
Well, yeah, but it's also 67 HP that has an AC of 9 and a move of 10'. So it might eat your blind gibbering buddy, or you might just all stand back and fill it full of arrows and have a good laugh about it later.
Now check the Gelatinous Cube. Very good chance for a surprise round, where it's a save with disadvantage or get sucked in and take 10 damage. If you fail another save, you're down, no question: 21 auto-damage. No one is going to stay up from 31 damage - most level 4 PCs won't. And if you stay in another round, you're dead completely from the 21 auto-damage again. And your buddies can try to help with the 2nd save but it's 10 auto-damage to anyone who does - that is ROUGH. But at least it's two saves, and they are DC 12 Dex then Str - most will at least have a good shot at one of those. And the thing has auto-hit AC. But 84 HP means it will definitely get many turns, and considering it is large there's no reason it can't do this to multiple PCs at a time, possibly as many as four. Quite a bit more scary - still CR 2. There is a good chance multiple PCs could get killed.
No doubt about it -- any day you walk into a gelatinous cube, that's going to be a bad day. For the poor sap walking point, anyway. Everyone else can point and laugh from a safe distance.
And then you have crazy combo CR 2s, like the Giant Elk.
You had your chance to learn Giant Elk so you could explain to them why they shouldn't be stepping on your face. It's hardly the elks' fault you kept falling asleep during Introduction to Giant Elk Love Poetry class.
So do the CRs seem a bit arbitrary?
Yeah, sure, a bit. It might be useful to have another number to reflect the variability of the monster -- if CR is the average challenge, some way of expressing the standard deviation might be useful to some DMs. But it might be more hassle than its worth.
Cheers,
Roger