TL;DR: If you look at the best stocks, measured by 5-year performance, they sometimes go way down within those 5 years (mainly because of recessions). Not very surprising.
Is it because the top decile stocks are higher beta than the bottom decile stocks?
Is it because of the selection of rebalancing periods? I.e. if the big crashes happen towards the end of a five year period then perhaps the bottom decile stocks have already dropped significantly and have nowhere to go while the top decile stocks have risen significantly and have plenty of room to fall.