Allarom, what you're missing is that the turnover is an aproximation. They explained a bit, but can't look for it now, check the Dev notes talking about loot 2.0 (iirc, Dev 10, 11 and 12).
That is, "turnover" in effect means number of kills. They chose that number in peds because somehow encompasses the medium-high echelon of the players, but is a combination of factors.
My primary guess is that the lvl of the mob is too big for your setup/efficiency/something. It is hard to put in words and is even harder to read, I would guess. If I would be at that tt return, I would re-analyze what I am doing.
If you're at 55% eff, I would guess you're using some UL. The problem is that you still have MU spent so I would guess is enhancers. The problem of enhancers is that they create only damage, not tt expense (not linear, that is). It would be an ample discussion about their opportunity and I am not the most qualified to carry it.
In other words, to use the example they gave, you could reach this turnover with (let's say) 50 proteron stalker per day or with 2000 argo young per day. The second case, at same ped spent, would have a higher chance of reaching 97-98% tt.
I don't think 55% eff is inherently bad, but it depends on some factors which we simply don't know but can try to guess. And from this guessing we're all building strategies, which have the main fault of being validated or not only after spending, and even then one is left wondering wether it was chance. But at this turnover can't be purely chance, you have some systemic fault in your playstyle which may or may not be reimbursed. And then we're back to guessing.
I don't believe that looter profession works as an universal factor, but on a mob basis, somehow comparable with the evader profession.