Hi there!
This is *probably* related to different schedulers and different "start of new day" calculations between AnkiDesktop/AnkiWeb and AnkiDroid.
The easiest way to fix it is probably to just grab the new
versions we publish as 2.16alphas here:
https://github.com/ankidroid/Anki-Android/releases where this is
all just handled correctly. And of course let us know if you have
any problems!
If you want to stay on 2.15.6 then you need to make sure in Anki Desktop you turn off "legacy time zone support" in settings, and in AnkiDroid you go to Settings -> Advanced and turn on "v2 scheduler" and "new timezone support"
That should get things in sync after the changes and a sync
Hope this helps -
-Mike