Good question and I do not know the answer. The only thing I can think of is that here is some sort of multitrack memory management going on. Reason I guess this is because there is only one wav file per loop track (on the Boss RC300 or 505), but you can undo last dub per track, until you do another dub, then it becomes permanent.
Not sure if this helpful, but the fact that the first loop does not get progressively quieter with new dubs, not does it get louder and you can always hear the dry instrument coming in is a key feature and have not been able to figure out how to replicate with the dry/ wet and dub settings in the ER301.
The other key feature is variable length loop, which think you can do now, you just need a reset marker set on punch outthatcould be routed internally