Here's my guess of what happened and why:
The day that you did the FILL, April 29 was still a "forecast" -- things remain "as this may happen" until the end of the day. So at the time you clicked on FILL, there was a forecast of 2.5" of rain. Because you were already over 100%, AND the system thought it was going to get 2.5 more inches of rain, in order to compensate, it had to show a -3.08 inches of watering to bring you DOWN to the 100% level. That number ACTUALLY gets entered in, just as if you had manual watered, that number would get entered in, but everything else (that pesky precipitation) remained a forecast. Then at the end of the day, the system recorded everything that actually happened. And you got 0.8" of rain instead of the 2.5". So now, based on "reality" figures, you ended up at 0% moisture balance.
Hopefully it makes sense when you think about everything for the day being a forecast, except for the watering that actually occurred. (and in this case it was a negative watering).
(Don't worry -- you're not the first one to do this -- I did it myself a week or so ago -- saw my ground was saturated, wanted to keep it from running, I was at 112%, did the fill and it went to 100% -- now it thought it was LESS saturated!! So I learned my lesson!!!)