IBU's don't seem to match up

I usually brew all grain big beers with OG's in the 80's and 90's. My last 2 imperial IPA's had the a similar hop schedule to the one below:

Columbus 1.5oz AA 14.80% 60 min. IBU 56.7
Columbus 1 oz AA 14.80% 25 min IBU 26.3
Cascade 1 oz AA 8.70% 5 min IBU 4.4
Cascade 1 oz AA 8.70% 1 min IBU 1
Columbus 0.5 oz AA 14.80% whirlpool IBU 0
Total IBU's 88.3

( sorry this looks all jumbled up in the post )

Both brews turned out great....nice balance of malt sweetness, hop bitterness and hop flavor....although I'm wanting a touch more of the hop bitterness. Also, I use whole leaf hops.

Lately when I drink some commercial beers that show lower IBU's, they taste more bitter than mine such as DFH90, Flying Dog Double Pale, and Terrapin Rye Squared.

Any suggestions on why this happens in my brews? Does my hop schedule look like it's calculated correctly? I also am using bottled spring water for my mash and sparge.

Probably a combination of things.

1. Maybe the commercial beers are finishing out drier, so that there isn't as much residual sugar left to balance out the bitterness. DFH90 in particular uses a constant hopping schedule which is almost guaranteed to taste different from what you are doing.
2. There's more to bitterness than IBUs...it has to do with the levels of alpha and beta acids that are extracted, and the types of hops and boil times can make a big difference.
3. Your hops may have degraded some since they were tested.
4. Also, the calculator itself is just an estimate and there's more than one way to estimate IBUs, which can result in very different numbers for the same additions.

i have had this same issue. wierdboy seems to have the answers