What is the secret sauce on iTunes Match deleting tracks?

I got a replacement iPhone 5 and decided to try the 32GB, having always bought the 64GB before, as I wanted to see if iTunes Match makes carrying around all my music unnecessary.

For the initial sync, I had iTunes Match disabled, and copied over a number of Playlists, and told (desktop) iTunes to fill the remaining space. That was about 15GB. Then I enabled Match, which immediately deleted 1.5GB, so I had to re-download the contents of some of the Playlists. In a perfect world, it would have deleted those filler songs first, but it's not a big deal.

Since then, the library has been hovering at about 13.2GB, or 4,722 songs currently. There will be a lot of stuff in that I never need to hear again, frankly. But I see it continues to delete songs from the Playlists that I have just recently (past few days) redownloaded, and in my mind "told" iTunes Match I specifically want. This is annoying: I care because I use underground public transit, fly quite frequently, and have capped downloads so while I certainly don't need all my songs all the time, I do want songs I plan to listen to, and have taken steps to make available, still on the device. Many hundreds - thousands! - of other tracks I may be pleased to be reminded of on shuffle are good too, but I don't get why many of them are surviving, when a Playlist I have very recently downloaded may not. I'd have imagined that a key variable was when a track was downloaded, but do I need actually to listen to it? Am I going to have to makePlaylists of things I might want to listen to before flights, and have them play with the volume turned down...?

So, gas anyone got to the bottom of which songs are selected for culling, and why?