Freeze plugs

The block was drained completely after the engine had been broken in. I guess what I don't get is, if it was slightly sideways in the bore, why it didn't come out when it was running?

I have a good micrometer somewhere in my garage. I'll find it and take a measurement.

What holds the plug in, anyway?

The plug is slightly oversized, so when you tap it in it is a pressure fit that holds it in place along with the sealer after it has dried down.