My guess is that the IBUs they are listing are measured, not calculated. All the formulas for calculating IBUs are imperfect since yeast, time, etc can pull IBUs out of suspension and process can effect how many actually get absorbed into the wort in the first place.
I'd go a few IBUs over, shoot for 20-22, and taste it side by side with an Old Brown Dog when it finishes. That way you can compare the bitterness at least. I'd also switch to grams for hops so that the amount doesn't look so small
, and its easier to measure more accurately on most scales.
OBD is a great beer. One of the first that got me into craft beer