In my opinion it is a combination of three things:

1) Delayed order confirmation (for example order was made when our working day ended so a couple hours there)
2) Variance (as you know you cannot mine every single block, however we compensate for certain amount of bad luck by crediting your balance from our own storage before the coins are actually mined - this is our so called insurance against variance - to prevent misunderstandings like this)
3) Around 0.001 would be consumed in electricity (our miners actually consume electricity (just a fact: over 5 years they would consume 3 times the contract cost))

I will be able to do the refund tomorrow - not in the office currently. Hope that's fine by you!

Best regards,
lunamine