Different routers can calculate the Line Attenuation slightly differently.

It is also true that some routers work better on different connections, depending on a raft of different factors, such as the specific equipment that you are connected to at the telephone exchange.

My connection changed when I replaced a faulty Netgear DG934G with the Sagem 2304n. The Netgear had always given me a Line Attenuation of 18dB. The Sagem reduced this to 17dB. The connection speed also increased to the full 20mb.