By a pure accident have found out that if you set incorrect time on your device it is possible to cheat Memrise streaks. Accidently my clocks were set on a previous day and almost all of my learning data on Sunday was added to Saturday.
It means that Memrise servers are looking at time settings on your device and syncing according to them, not working by themselves. It gives you an opportunity to prolong your streak in case you missed a deadline.
This is how it affects your stat