some songs re-downloading on sync

[TroubleClef]

When I sync my playlist from MMA back to MMW, some songs re-convert and re-download, and others don't. I used to think it was related to changing the rating in MMA before syncing, but now it seems clear those files don't necessarily always re-download, while others that were only played and not modified do re-download sometimes.

What controls this process? It would be nice to minimize the timely re-downloading process if at all possible...

[Lowlander]

Which version of MMA and MMW?