I used the Rager Formula and for 1.061 wort i came up with ~8g cascade and ~6g galena for 21 IBUs. 6g and 5g will get you to ~18 IBUs.
I really think this measured vs calculated issue.
A few grams for you is probably a couple pounds on their system, so they have the ability to use smaller...
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...