differences in IBU's calculations...whats going on?

I'm not quite where this thread should go, so i'm sorry if this is in the wrong place.

i've noticed a huge difference in the calculations from different websites and softwares in IBU calculations.

heres my recipe

3 lbs Light DME
3 lbs Wheat DME
1 lb Flacked Wheat

1 oz cascade 60min 5.1 AAU
1 oz amarillo 15min 8.0 AAU
1 oz amarillo 5min 8.0 AAU

wy'east 1272 American Ale 2

OG 1.054
FG 1.013

from beercalculus.com i should have 32.0 IBU's
from tastybrew.com bitterness calculator i should have 26.1 IBU's
from tastybrew.com whole recipe calculator i should have 44.5 IBU's
doing it by hand with the hops utilization chart in The Complete Joy of Home Brewing i get 32 IBU's

how can there be this much variation between them all, especially tastybrew.com, i mean, why wouldn't they use the same math in there two calculators, there should only be one right answer right?

the main reason i am worried is because i took a small taste of the wort after it had cooled and it was horrifically bitter.

is this gunna be a nasty batch?

And according to Promash, it's 17.2 IBU's

I wouldn't worry about the bitterness immediately after cooling. That'll go away over the next few weeks.

I would have used "wonderfully" rather than "horrifically" when describing the taste.

-a.

The calculations vary because they all use approximations to the real utilization curves. Some of the software out there doesn't 'kick in' until 10 minutes, but a fair amount of bittering occurs in that interval. The impact of SG is non-linear and software typically use linear adjustments. Most of them do not adjust for saturation, any IBU number over 90 is suspect for that reason.

Very few people can detect a bittering difference of less than 10 IBU, so these approximations don't matter.