What thief are you talking about? The pre-made character (THIEF.CHR file)? If so, those are not the original stats. That thief is a halfling called Dernel, and his stats are 50, 40, 35 and 35 respectively. Still ridiculous for level 1 though.

Yeah the import character thief. Not sure why mine is different, I'm not using any mods. Only thing I can think of is that it's because I have the GOG version instead of the disc.

http://i63.tinypic.com/qq3g34.jpg

Imoen in the Baldur's Gate 2 tutorial has ridiculous thieving skills as well.

Xan in the BG2 tutorial has 25 Intelligence.

You can use these characters in Throne of Bhaal if you import a tutorial save.

Ah yes, those figures are calculated due to his Halfling bonuses and his 19 Dex bonuses. That makes the original figures even more insane.

As Hickory mentioned Deder from Candlekeep tutorial has stats way higher than he should. Similarly other party members from that tutorial have some neat stats and stuff:
- Arkanis has 9 hp more than allowed for his lvl (2 lvl fighter with 19 con - 39 hp), with 18/53 str he's an amazing and durable fighter. Even more tanky than Kagain (unless Kagain will have 21 con with the tome).
- Canderous is the only cleric to have 18 wis, his 16 str also helps. He also has lay on hands ability (like paladins), but for some reason loses it after leveling up.
- Osprey being cleric/mage has a lot of spells at her disposal, her stats are not as overpowered, but she has a wand of the heavens (extremely useful at the beginning) and an invisibility as a special ability (similar to Quayle).
- Mordaine is similar to Dynaheir, has the same special ability (slow poison), but has 18 int and overall better stats and a wand of magic missiles.
- Deder is similar to Montaron (better dex but worse str), but his thieving abilities are absurdly high for his level.

You can actually have them permanently in your party, by using a trick with pausing before the dialogue. They're not very talkative, though.