I made some mistakes yesterday:
MacDive created a new tank because I (using Metric) don't care about Working Pressure and left it at the default 0 in MacDive and the default 205 bar in Dive Log. If WP's are the same (along with the other things), then the sync process matches tanks.
I also missed the gas mix field in Dive Log. So MacDive created a new mix with 0% oxygen. (No wonder I was out of breath!

Benthic