What on earth is my Explore 2 syncing?

I bought an Edge Explore 2 to replace the failing Oregon 700 I'd been using for years. Generally I much prefer the Explore, however,  as soon as I'd save my ride in the Oregon, a notification would pop up saying that my ride was available to view on Connect and I'd turn the Oregon off.

With my Explore 2 it's currently taking around 5 - 10 minutes for this to happen and if I drill down through the menus to Phone it shows that it's syncing - but what? Surely all it needs to sync. is the last Activity which is generally around 250Kb (even if it's moving everything in the Activities folder this is only a smidgen over 3 Mbs.). By contrast if I plug the Explore 2 into my PC via usb, it syncs with Connect (through Garmin Express) in a matter of seconds.

Having to wait  for the Edge Explore to finish syncing. is a bit of a faff, but if I don't, the  Connect App on the phone won't automatically upload the new ride to Garmin Connect and I'll need to sync. via usb. What's going on?