I think it's probably due to the Billion having a Broadcom chipset.
See the link in my signature - a Broadcom chipset (I use an old Speedtouch 585v6) transformed my line.
You might want to post your line's attenuation as that will help people see what your line is capable of - but it sounds pretty healthy to me
According to several people on the t'internet the 7300N has a 3 part Trendchip chipset, the 7700N and 7800N have Broadcom.
I'm not going to prise it apart mind you, (well not while it's under warranty and still working). My previous Netgear 2200N had a Broadcom chipset but tried too hard and used to disconnect often.
The Billion reports my attenuation as 45 dB, other routers have reported between 44 and 48 dB at different times.