There can be several reasons to fail this step, mainly connection issue (or quick alt-F4). Or computer issue, as this step lasts 10s for now. If the user was playing in solo game and has a slow computer, it could take more than 10s to unload this game and authorizes the synchronization, which means "ok now, let's start the game, send me game data like my opponent, the chosen map and my units to start the adjustment step as I'm ready and able to display that screen".
Aren't you confusing synchronization and adjustment ?
The window to select your army within 60s is the adjustment. The synchronization is another popup displayed before that (which generally is almost not visible as it lasts a very few frames). During the synchronization, you have no idea yet of your opponent.
People are not really supposed to refuse during the synchronization step
. The first popup to accept/refuse the game has the yes/no choice. After that, you just play : adjustment, deployment, etc..