As I noted earlier in the thread, the lower output started with the CM11A (the CM15A is a little better than it). Europe limits PLC signals to 5Vpp and X-10 likes to use the same basic design (aside from line voltage and plugs) there as in N. America so that may be a factor. I'm unaware of any limit in the US although there are limits on unintended conducted emissions which accounts for the signal suckering capacitors across the mains that manufacturers have added to so many non-X10 devices. I have not done an extensive study of FCC rules on this point so there may be something I missed.
AFAIK, the FCC limits cover 150kHz-30MHz so X-10 (and Insteon) would seem to be exempt. However, there is an International standard, CISPR 22 which has been adopted in much of Europe, which applies from 9kHz-30MHz. I have no idea whether it applies to N. America. It may be that X-10 expected CISPR 22 limits to eventually apply in N. America - the 9kHz lower limit dates to 1993. When was the CM11A introduced?
Also, units from other manufacturers introduced after X-10's patent expired (Stanley Homelink 30001, Leviton HCPRF, Powerlinc) tend to have similar outputs to the later X-10 units which may be another indication that they expected regulatory tightening.