Your understanding is correct Jonathan. I am not sure what the reason is.
USB isn't really well suited to these kinds of loads, because other devices on the same bus can degrade performance in a way that seems random to the casual observer.
If you mix USB 1.1 and 2.0 high speed devices, you're also asking for trouble.
Also, the max theoretical speed of USB 2.0 high speed is 480 Mbps, or roughly 50 MB/s, while SATA I is 1500 Mbps (150 MB/s) and SATA II is 3000 Mbps (300 MB/s).
Other ethernet enclosures from different vendors could perform better than the ethernet Buffalo offerings, but I don't remember even seeing test results of an ethernet RAID enclosure that showed figures even remotely close to 125 MB/s transfer rates.
125 MB/s over single 1 Gb ethernet is guaranteed to be a blatant lie.
It's just 1 Gb divided by 8. Even at optimal performance, a typical storage protocol over ethernet would not use 8 bits per byte for transfer, but at least 10. So the theoretical best you should get, would be 100 MB/s. Then there may be additional overhead.
Add in file system characteristics, and you'll never see the theoretical performance of these external drives, just like you never see the theoretical performance of your internal drives.