So if your usable fuel was far significantly lower than you expected, how would you respond?
I know from my own near fuel exhaustion events (one, an error of not swapping tanks, the other due to unexpected low-level flight due weather, then headwinds, then unusable landing sight, then more headwinds, then crazy cross-winds) I am now very conservative in this respect.
However, if I "filled to full" but my actual usable was "full less X" where X is significant, that would be a serious WTF event.