Why do my 2nd runnings make up for my lack of first runnings?

Specifically,

I like to try and estimate what I should get based on grain absorption, initial volume, some wiggle room for evaporation etc.  I'm often way low on my first runnings, sometimes as much as a half gallon.  However, low and behold my second runnings is always more than what it should be?  For example, my first runnings last time was around .5 gallons short of where it should have been.  However, the second runnings was (surprise) around .5 gallons more than what it should have been (I actually got more than what I put in?)

I guess its not that big of a deal but I was wondering if anybody might know what is going on?

My best guess would be that the "water absorption" factor of the grain trapping some water largely occurs in the first runnings, while the second runnings achieve a higher "flow thru" since the grains are presumbably fully saturated by then. 

Maybe since that must be such a fuzzy-math type equation, (equipment, water temps, grain crush, etc.) BSmith has difficulty estimating it?