Those last two guys seem to be talking about a different thing than I was - all I noticed was some potential weirdness around the expiry date where it seemed to reset the unused remainder to the full initially offered duration (or possibly just rounded up to a whole number of days; would have had the same effect) whenever funds were moved from unused to used. Never noticed any discrepancy with the payment.

I wonder if it's connected with that other oddness in the other direction, where the 'time remaining' on reserved funds repeatedly ticked downwards as they were used... could be the implementation only has a concept of integer numbers of days.