Actually, a,b,c are also sampled from the range of valid bit indexes (not uniformly on 0..127), so you might be able to pick a cardinality-72 subset of 0..127 such that random a^b^c is disproportionately likely to fall outside that subset (and thus get diagnosed as not a valid single-bit error correction). I don't know that any existing ECC implementations actually do that, though.
Edit: did some cursory testing and using indexes 0..71 actually catches only ~24.04% (86016/357840) of triple-bit errors, compared the theoretical 43.75% (156555/357840, I think?) from a random error index. So "doesn't change the probability" is just completely wrong given that a,b,c are randomly chosen from the 72 substrate bits, rather than from 128 possible 7-bit indexes.
Oddly enough, testing random selections of 72 valid indexes (eg 74773982'EBD0D35C'C5BEB2D8'C3FE9C5E, where set bits correspond to used indexes) actually gives slightly better results than theory (44.97% for that one, 44.65% is the lowest in the last dozen or so), which is somewhat interesting, but I still haven't found any bit assignment that gives better than 50% (178920/357840) catchment of triple bit errors.
Actually, a,b,c are also sampled from the range of valid bit indexes (not uniformly on 0..127), so you might be able to pick a cardinality-72 subset of 0..127 such that random a^b^c is disproportionately likely to fall outside that subset (and thus get diagnosed as not a valid single-bit error correction). I don't know that any existing ECC implementations actually do that, though.
Edit: did some cursory testing and using indexes 0..71 actually catches only ~24.04% (86016/357840) of triple-bit errors, compared the theoretical 43.75% (156555/357840, I think?) from a random error index. So "doesn't change the probability" is just completely wrong given that a,b,c are randomly chosen from the 72 substrate bits, rather than from 128 possible 7-bit indexes.
Oddly enough, testing random selections of 72 valid indexes (eg 74773982'EBD0D35C'C5BEB2D8'C3FE9C5E, where set bits correspond to used indexes) actually gives slightly better results than theory (44.97% for that one, 44.65% is the lowest in the last dozen or so), which is somewhat interesting, but I still haven't found any bit assignment that gives better than 50% (178920/357840) catchment of triple bit errors.