In short-some of that comes from searching is:sent vs in:sent vs actually clicking/browsing the sent folder. Personally I think in:sent or clicking the folder should show 'To:' while is:sent would not change the layout. Thus once you open the convo it should be 'to' instead of 'from' - however it currently does not (in RC1 that is):

-unless a zimbra employee can verify that this is already changed on their current build - as C21 talks about this but it doesn't mention if it's been fixed this way