iirc I thought we'd already noticed that your 'R' value was higher and therefore the level of redundancy would be higher, which would in turn affect the overhead allowance?
It's unrelated to R. The FEC overhead has already been subtracted to get the "sync speed" (Net Data Rate).
I think non-retransmission, on the high FEC+interleaving setting, can have INP 8, and if they have a higher proportion of FEC data, then the Net Data Rate will be lower, but the IP Profile will still be the usual approx. 96.7% of the Net Data Rate.
There is a table giving the parameters of the retransmission profiles in SIN 498, in section A.4.2.2.8, although the table is possibly incorrect (it gives INPMIN_REIN_RTX of 1 for both low and high retx profiles) or at least does not correspond to what's actually in the live network. But it's possible that the lower 91% IP Profile is factoring in the higher SHINERATIO_RTX.