'Which factor(s) made this flight so special?'
Doesn't the answer come from page 17?:
Analysis of fuel flow from the 13,000 flights shows that 10% had fuel flows less than 10,000 pph during step climbs (the accident flight did not exceed 8,896 pph), and 10% had had fuel flows greater than 10,000 pph during the approach phase (the accident flight was greater than 12,000 pph). Although these were not unique, they were at the edge of family for the data analysed. However, when analysed in conjunction with the fuel temperature data above, all of these factors make this flight unusual within the 13,000 flights analysed.
Maybe we should infer that the absence of high fuel flows in the cruise (gentle cruise climbs) promotes ice formation while the high flow on descent encourages any slug that has formed to move; take away either and you break the chain. Since, as others have said, it isn't obvious that there is anything special about the RR engine in all this, maybe this analysis will be continued to the other 128000 PW and GE powered 777 flights. If BA38 is still ' unusual' in the bigger data set, that would be more than interesting.