The calculator has had that problem for as long as I can remember, well back into PMCO days. Given the bug does not benefit the customer (nor, fortunately, hurt the customer!), it is not going to be high up UA's list of things to fix: they've got to first take care of 4 RDM awards to HKG, clawing back "Map Your Nightmares" bonus miles, and eventually stripping incorrectly-awarded premium cabin bonuses for upgraded flights (I'm wagering on 12/31 for that to happen).