[erockrph]

What temperature did you measure your mash/sparge water at? Water expands as it heats up. If you measure your initial volumes cold, then measure your runoff hot this could account for some of the difference.

[ynotbrusum]

Some very logical answers, because I measure volumes only at initial strike and sparge - all at ambient temperatures - it could be both the temperature and the added gravity of the sugar becoming rinsed out with the second runnings.  Never thought about that....so I'll save the magic answer for another topic!