Looking at your image, you may have slush as your main pool, but your S1 has also been mining elsewhere during the session it reports a block as having been found (i.e your second pool). If indeed one of your secondary pools is an altcoin one, then that is where you found the block (as clearly your best share is not big enough to be a BTC block).
Additionally, I do not believe that a stale share can also be flaged as a found block, so that is that with that.