Out of curiosity, can you or anyone else think of any reason this could be that would have anything to do with the layering itself? I can't. If you took a jumbo marshmallow and cut it into three pieces (the cuts being the same direction as in layered tips) and then glued it back together it would still compress just as easily. One explanation could be that layered tips generally start out fairly hard to begin with and therefore don't change as much in their hardness. My best guess is that when glue is applied to all the thin sheets of leather and then they are all pressed together to get a good glue bond this also compresses the leather some. So just like milk duds, layered tips already come pre-compressed to an extent (and this may be why there really aren't any soft layered tips out there even though some are called "soft" by the manufacturer).
If this is the case then it would seem you could get the same benefit with single layer tips by just compressing the single sheets of leather they are made from in the same way the layered tip leather sheets were pressed. Maybe compressing the individual single layer tips would even work. I'm talking dry pressing, not milk duds, but then again maybe the layered tip leather is treated with something before it is pressed just like milk duds are--I don't know. Can anybody that has dry pressed single layer tips before installation give their feedback on the results?