As i said above it was probably because the algo wasn't optimized @ all for new hardwares or by itself, lbry was niche altcoin not long ago, for very optimised algo (eth for example), you will get maybe a few % from private not more.
You are right, this was in the begining. But - remember darkcoin(Dash now) and his x11 algo? All started from 4mh on 280x, after some time it increases to 6-7mh on public miner, and after year or two(if I not wrong) - nicehash upload "leaked precompiled bins" with 12mh performance. How long time some people used these optimized bins, while all others used "normal" bins? Month, two? Or more? I dont know...
And for eth - there exist
private miner , from Wolf too, that gives +10-15% more hashrate. I think, that with custom memory timings, it is possible get up to +25% on comparison with CM/SG and standart memory timings mod.
So, some people has much more profitable mining, than others...and they are among us.