They did, but it wasn't fantastically less xp than the Cleric. It got even stranger with the 2e Bard...but here's the problem. The Thief gets the second worst attack bonus progression, and 6 sided hit dice. The way the xp tables were set up, you'll be about a level higher than a Fighter for awhile, then pull ahead when you hit 9th before they hit 8th. You'll probably hit 20th when the Fighter is 16th.As I recall, the Thief in AD&D took the least amount of experience to level up precisely because they were the weakest class. Am I remembering wrong?
But here's what that gets you (going off of memory mostly)-
Level 16 Fighter, average hit points (w/o Con): 73.5
Level 20 Thief, average hit points (w/o Con): 55
Level 16 Fighter, hits AC 0 on a 5 (w/o bonuses).
Level 20 Thief, hits AC 0 on a 11 (w/o bonuses).
Add in the fact that Fighters get higher bonuses from Strength and Constitution (and Thieves want Dexterity high because of Thieving abilities and crummy armor), better weapons, better AC, and making 2 attacks per round as opposed to the one of the Thief, and you can see that there's a lot more going on here than one might suppose.
Sure, by this point, you have very high numbers in most Thieving Abilities (with I think Read Languages being terrible in 1e, though 2e let you adjust your scores), but even if you can successfully sneak up on an enemy, the restrictions for actually using your Backstab were atrocious (must successfully Move Silently, must attack from the rear, enemies cannot be aware of you, you must be able to reach a "vital area"). You do get +4 to hit for the rear attack, and multiply the base damage of your weapon by 5 (probably a d8 from a longsword) but then you better hope that kills the thing you attacked, because you're going to be a good distance away from your party!