Well there's a few factors here...
1) The types of versions - the USB version is not as new and exciting for us as it was back when NL was released. Sure some of up got more than version anyway but it's not like this is the first Ayu USB and we don't know if another one will ever be made... RnRC had more versions too, if you count the box sets, which I think there were 2 types of, and that the CD+DVD in the box was slightly different to the regular one (gold vs red obi)
2) Handwritten lyrics - no handwritten lyrics so people aren't buying multiple CD Only in case they get the lyrics
3) Economy - The US economy has kinda sunk in the last 6 months. I'm not surprised if less Americans are buying copies right now
4) Speed of releases - NL > RnRC's release dates are longer than RnRC > LS. Some people might only be able to get 1 album a year or just aren't ready to get LS until they get full value from RnRC.
5) People aren't bothered to add their purchase to the count? I don't think I ever added my CD+DVD purchase of GUILTY
AHS has lots of members, sure, but it's not representative of ALL of Ayu's international fans!
Ayu's probably got 4-5 times the amount of active AHS people who buy her albums. Let the sales speak for themselves :3