Ahh, that explains a bit: let me guess, your modem had a single Ethernet port on it, which you were running directly to your computer? That means the modem was

Ahh, that explains a bit: let me guess, your modem had a single Ethernet port on it, which you were running directly to your computer? That means the modem was cloning your device's MAC address onto its own WAN interface. Which is... weird, but would have had some kind of logical motivation. Maybe it meant the modem vendor didn't have to maintain their own MAC registration ("just copy the device's!"), or maybe it was an ISP "what kinds of devices are our customers using?" kind of thing (the majority of results would have been routers, not single devices). Or maybe this is normal for cable modems (never used cable myself, just ADSL2+).