Probably true ... I would hazard a guess that it approximates to where D2 starts to run out of capacity. As lines get longer, uses will initially lose the top of D3, then as D3 is eaten into U2 starts to be affected. When D3 is exhausted, possibly around 50 to 60 Mbps or so, there will be very little drop off due to length as U2 is at the top end.
If Max Achievable stats are looked at there might well be a "sweet spot" somewhere around the 50 to 55 Mbps mark.
What it might also do is to cause some users to downgrade from 80/20 to the 55/10, if they are only getting low 60s.
M H C
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