iFA88 , this is because of how the system is set up on this pool: each share gives you a score that slowly falls down if no new shares are submitted. In effect, the shares have a set lifetime; when they "die" so does that little part of the score they brought.
To exemplify, let's forget about the score and only consider the share lifetime.
Let's say a block is found in 10 seconds, and you submit 10 shares, each share 1 second apart:
Time 0s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10s
Shares 1 2 3 4 5 6 7 8 9 10 Block
We're also going to presume each share has a 'lifetime' of 3s. This is how the system works.
So, this means that if you mine for 2 seconds then stop, you would have 3 shares. However, since the block is found at 10s, those 3 shares will look like this:
Time 0s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10s
Shares 1 2 3 3 2 1 0 0 0 0 Block
(share 0s dies at second 4, share 1s dies at second 5, share 3s dies at second 6)
So at the end of the shift, when the block is found, you do not have any relevant shares, although you have submitted 3 shares.
The opposite case is when you start mining towards the end of a shift:
Time 0s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10s
Shares 0 0 0 0 0 0 1 2 3 3 Block
At the end of this shift you would have 2 shares (share 6s would die right as the block was found).
P.S. Any chance we could add an explanation like this to the pool FAQ to avoid having this sort of questions every once in a while?