My Standard Catalog of Firebird states that in '90 and '91 there were two LB9s offered - one with the 700R4 and one with the T5. What confuses me is that each has a different hp and torque rating. All other years ('82 - '89 and '92) only list one LB9 no matter which transmission is used. According to the book, the engines differ as follows:

'90 LB9 with 700R4: 200hp @ 4400rpm - 295ft. lbs. @ 3200rpm
'90 LB9 with T5: 225hp @ 4600rpm - 300ft. lbs. @ 3200rpm
'91 LB9 with 700R4: 205hp @ 4200rpm - 285ft. lbs. @ 3200rpm
'91 LB9 with T5: 230hp @ 4400rpm - 300ft. lbs. @ 3200rpm

All have a CR of 9.3:1

Were the only differences the cams used? And why only those two years? The reason I ask is because I have the '91 LB9/T5 combo in my '86 T/A. Of course mine now has an LT1 cam so my numbers will be different. Just curious if anyone knows why the differences.

Besides the cam, the engines are the same. Some cars either came with or could get the dual-CAT exhaust option.
Even with identical engine output, automatic cars generally show lower numbers on a chassis dyno than those with manual trans.

I assumed GM dynoed the engines out of the car to achieve higher hp numbers. Or did they just correct the numbers after the fact? And why then where the numbers only different in '90 and '91?

It looks like they were correcting for the extra power taken up by the automatic tranny. Just an opinion though.

Gm installed different cams (even in earlier year F-bodies) for auto and manual LB9 cars. Auto's received a smaller profile cam, while the manual's received the more aggressive L98 cams.

305 TBI and 305 TPI auto cars shared the same peanut camshaft, while LB9/M5 and L98/A4's had the more aggressive camshaft.

Even further than that later you can get the dual cat options and which bumped the #'s up a bit too.

The only exception to this rule that i know of is the 1985 305 TPI cars because it was the top model and GM installed the L69 camshaft in those which was very close to the later L98 camshafts.

Even among the different years the L98 camshaft changed profiles slightly. 87-89 got a higher duration cam, but the lobe separation was wider so it had a smoother idle with less top end. And the 90-92's got slightly less duration, but a smaller lobe separation. End result is that both are very similar and I think the 90's got slightly more power. Perhaps the early MAF ecm's were having issues with a smaller LSA and so GM developed a cam with a wider LSA to make the ecm happy.