The 37.8 IBUs came from the higher hop utilization due to the lower boil gravity. Hop utilization is a function of boil time and boil gravity. The question, from How to Brew is:
U = (1.65 * .000125^(Gb-1))((1-e^(-.04 * T))/4.15)
Where Gb is boil gravity and T is time in minutes. If you crunch the numbers for each hop addition, you get:
U1 = (1.65 * .000125^(1.047-1))((1-e^(-.04 * 60))/4.15) = .237
U2 = (1.65 * .000125^(1.047-1))((1-e^(-.04 * 30))/4.15) = .182
U3 = (1.65 * .000125^(1.090-1))((1-e^(-.04 * 10))/4.15) = .058
The 60, 30 and 10 are the different hop addition times and the 1.047 and 1.090 are the boil gravities at the time of addition assuming a 3 gallon boil. Now plug those numbers into the IBU equation and add them up to get your total IBUs in the boil:
IBU = (AA * U * 74.86) / V
Where AA is the alpha acid percentage of the hops, U is the utilization we just calculated, V is the volume of the boil and 74.86 is a conversion factor to convert English units to metric. Typical AA for Willamette is 5.5%, EKG is 5% and Fuggles is 4.5%.
IBU1 = (5.5 * .237 * 74.86) / 3 = 32.5 IBU
IBU2 = (5 * .182 * 74.86) / 3 = 22.7 IBU
IBU3 = (4.5 * .058 * 74.86) / 3 = 6.5 IBU
IBU1 + IBU2 + IBU3 = 61.7
Now, since you'll be needing to add top off water, from 3 gallons to 5 gallons, you'll reduce your IBUs by that ratio.
3 / 5 = .6
61.7 * .6 = 37.0 final IBU
Wow, that number is pretty off from what BeerSmith says it will be, but the point of this wasn't to prove BeerSmith right or wrong. I just wanted to show you how to do it by hand so you understand where all the numbers come from and what's going on behind the scenes when a calculator calculates your IBUs. There's quite a bit of other math involved in brewing, but that's a different topic for a different day.
Whew, that was quite a bit of writing and number crunching. Anyone think I should put this on my blog, or is it common enough knowledge?
Best of luck on your brew!