Actually, it should be possible to create leather armors with AT11 or 12, since some items are listed in RMSS Treasure Companion (Section 16.1.2, 11th-20th level, dragonskin armor). One is AT12 with +15 DB and encumbers as AT10, the other is AT11 with +15 DB and encumbers as AT9. There is also an AT3 dragonskin armor (shirt) that gives +10 to DB and does not encumber at all.
Presumably, one would need alchemy to work the caveworm hide, basilisk hide, and dragonette hide to create those armors. But it can be done. I would assume that level 18 spell 'work magic leather' would provide the capability to work the hide, and armor III or armor II spells would provide the DB bonus. It is a bit difficult to know where the reduced encumbrance comes from - my guess is that it is because they are shaped as AT10 and 9 respectively (and as such have the corresponding encumbrance penalties) - the increased AT is not a function of coverage, but a function of material.