If you look in EKAT, you'll see that the ETS125, DTS400 and RTS400 share the same housing, armature, field assembly brush holder, and cam. In fact, I counted only 3 or 4 parts different between them, and all of them are related to the different plate size (plate, collar, etc). Nothing movement related is different. When you compare the critical parts of the ETS125 to the ETA150's, you'll see that they are ALL difference.
So really, the ETS 125 is the same sander as the DTS400/RTS400 (only difference being pad/plate size), and those are cheaper than the ETS150 to begin with..
The ETS125 should probably be called ETS400, which would cause a lot less confusion when people look at pricing. That being said, the ETS125 is $60 cheaper compared to the DTS/RTS, and that difference isn't made up in the plate/pad cost differences. In fact, the backing pad for the ETS125 is $10 more than the pad for the DTS/RTS! (all prices based on US prices)
So, ultimately, I say it's purely a marketing thing.