Had some coins stake last night and can now explain the rest of it.
First of all here are some coins in my wallet of the correct age for POS.
http://i115.photobucket.com/albums/n316/tmuir1/Cashcoin/coincontrol5.pngThen during the night I had two lots of coins stake and earn me some interest.
http://i115.photobucket.com/albums/n316/tmuir1/Cashcoin/coincontrol6.pngI'm going to explain what happens in your wallet now focussing on the one that gave me 0.128134 Cash
http://i115.photobucket.com/albums/n316/tmuir1/Cashcoin/coincontrol7.pngHere is the same coins, note the amount of 10.054841 is no longer there but there is an amount of 5.091488 and 5.091487
If you add these two amounts together you get 10.182975.
Now if we take the interest 0.128134 from 10.182975 we get 10.054841.
So when your coins are used for POS the interest is added to the total and then this total is split in half and the coin age is reset to zero.
This means every time you earn interest the amount used is broken into two halves which means if you do nothing you POS will be roughly half for each payment, but you will get twice as many.
If you don't like this idea send them back to yourself to group them back together to get fewer but larger payments.
If you do the math you will see the coins earned just over 1.27% interest as there coin age was around 66500, or just over 46 days, so 10%PA interest was paid on just over 46 days, proving you don't loose interest if the coins don't stake right at the 30 day mark.
Hope this is of use to some people