The Amazon pre-order page for volume 5 says March 13, so I'm guessing the printing simply got pushed back a bit from the scheduled release. I'm sure we'll see it on Akadot/Yaoi Club soon and other online retailers a few weeks later as usual. As for the kindle edition already being out, I think they've been releasing their digital editions before their print editions (on the titles for which they have digital rights) by a few weeks. It's true that reviewers get their review copies digitally weeks before the print editions are out, even at Akadot. They probably put things up online when they have them finished, and send them to the printers at the same time, but because of the time it takes to print and ship the books back, the print edition doesnt come out right away.
I'm incidentally really looking forward to this release. It's the only thing holding back my next Akadot order.