I've very little trust in whatever algorithm it is that computes one's "attainable" speed. I think it is a confabulation between the modem and .. something else, like the exchange.
But I'd be delighted if someone could persuade me it means something.
My DS attainable is a shade below 100Mbps. (I've lost a whopping 3Mbps in the last 9 months!) It syncs at 80Mbps. I get in reality data throughput around 72Mpbs .
My US attainable is about 22Mpbs, It syncs at 20Mpbs. I get in reality about 18Mbps. (I'm on an 80/20 Plusnet FTTC contract.) All SNRM's waggling around 7dB.
My explanation, to myself, is that my DS is massively interleaved (INP 16, INP 48, 20-something ms latency)), and that about 3/4 of my DS throughput is the transmission of error-correcting/detecting bits. (My line is terrifically stable.) My US, about 1/4 of the DS, doesn't bother so much to correct things, so is the main source of the errors my line experiences. I only got these numbers after capping my line for several months.
Anyway, what does "attainable" mean? Is it remotely interesting, or even meaningful?