You might be looking at the average mma purse (which includes the lower level circuits like Legacy and Bellator, and not the average UFC purse. That article said the average UFC purse was $61k, while the average boxing purse was considerably lower than that.
Additionally, a UFC fighters $20k show purse means he gets $20k just for being there if the other guy has to be removed from the card; meaning if the fight is scratched he still gets his $$.
Their minimum fight purse is $40k plus a $5-10k endorsement purse from Reebok, win lose or draw.