Can you think of any scenario (apart from errors of the measurement tools) where gravity actually goes down during boil. Normally, due to the loss of water, gravity should always go up. During our last brew session, we measured lower post-boil gravity (1.058) than pre-boil (1.068) and we're quite puzzled.
How we took the samples:
Pre-boil: pipette from top of stirred boiling pot.
Post-boil: pipette from first drawings after whirlpool.