I don't know why, and I haven't been able to figure it out in any of the other brewing software applications either, but it is impossible to reach according to scientists. The simple fact is that a wort cannot have more than 100 IBUs regardless of gravity and volume, and if you take 2.5 gallons of 100 IBU wort, and dilute it with 0 IBU water, then you are left with 50 IBUs, and that's the most you can ever hope to get.
Also, BeerSmith is initially set to use the Tinseth formula for IBU calculations which is based primarily on full boils, the Rager formula is geared more towards extract brewers, but still does not take fully into account IBU limitaions. You can change BeerSmith to use Rager or Garetz instead.
And to completely answer your question, there is no "true" IBU, those three formulas I mentioned can give you huge (30+) differences in IBU calculations, even on the same recipe. The only way to really find out "true" IBUs is through photospetromic analysis.
Find an equation that works for you, or that you get most of your recipes from, and stick to it, but realize that you taste buds are going to be better determinants of IBUs than calculations.