Really, they are affectionally called baby-jumbo frames. Instead of going up to ~9000 bytes that "real"jumbo frames would allow, some ISPs allow up to 1508 bytes so that the PPPoE overhead does not eat into the MTU to normal internet sites (which typically are restricted to MTU = 1500). So one still does pay the 8 byte overhead out of the link bandwidth. but at least one can use MTU 1500 packets to other hosts on the internet.