Rewind Question-Why am I sometimes able to rewind immediately after tuning to a channel

Ok, this has happened to me a few times now and has me confused.  Today, I was watching recorded DVR shows for a while, then changed to Velocity for about 40 minutes.  I then changed to A&E, which I had not had on at all today, and I was able to rewind the show as it had the last hour buffered.  I was not recording anything at all at the time.  I haven't recorded anything at all today.

The DVR'd shows I was watching were recorded from A&E though, but like I said, I went from DVR to a live Velocity program for about 40 minute before I switched to A&E and was able to rewind.

Why was I able to rewind a channel that I had not been on at all today?  This has happened before on different channels as well.  Typically once you change a channel, then go back to the original channel, all video that is buffered is lost.  Why am I able to do this sporadically?  

It would be great if I could do it all the time, but I don't think I'd get that lucky.

Thanks.