Tastybrew.com discrepancy

Im getting different figures for IBUs from the bitterness calculator and the total recipe calculator. Stats as follows

Recipe volume 8gallons
Boil volume 2.5 gallons
4.5 lbs light DME
3.3 lbs corn sugar
.75 oz Admiral 60 min
.25 oz Admiral 15 min
.5 oz Hersbrucker 15 min

Recipe calculator says 28.7 IBUs
Bitterness calculator says 11.4 IBUs

Is it the case that the recipe calculator doesn't correct for utilization according to the boil volume and gravity data? If so how can I correct this and future extract brews to hit my IBUs?

Boiling at gravity of 1.000 returns 40.3 units in the bitterness calculator, so the recipe calculator seems to be adjusting for utilization to return 28.7, but the formula would seem to be different to that in the bitterness calculator.

If I'm reading this correctly, you're making an extra brew where the final volume into the fermentor is 8 gallons, and you're boiling 2.5 gallons?

Correct, I have one smallish brew pot, and a large fermenter, hence the small boil and concern for blocking the utilization. Btw the brew is in the can as of yesterday, I only discovered the discrepancy after the fact.