Before this, everything was synchronized, on desktop and mobile.
Then, I added a new deck on the desktop and added 9 records, they are all pure text records with less than 100 characters each.
Before closing the desktop Anki, I accidentally opened the Android Anki.
It started syncing automatically after starting up.
Then I closed the app without doing anything else.
After going back to the desktop Anki, when doing syncing, it asked me whether to use the local version to overwrite the Ankiweb version or the other way around.
I overwrote the Ankiweb version with the desktop version.
It was really quick.
Then on the Android version, after starting up the app, it also asked me which side to keep. I chose to keep the Ankiweb version.
Then the syncing started, but it was very slow. More than 200MB of data are received. I waited and waited for a long time. At last, there was an error saying something like some media files were not synchronized properly…
Why does Anki need to update so much data, as all I changed was a new deck with 9 records?