Many rate film at an exposure index other than than the "official" ISO speed, for different reasons. Some arrive at their preferred EI through testing, and others (like me) just make adjustments until we get results we like. When I was a QC tech at a commercial portrait lab, we told our photographers to rate Kodak Vericolor III (ISO 160) at EI 100 (I don't recall the stated reason). I often rate Tri-X at 200 instead of 400, and develop normally, because it helps me get better shadow detail. When I shot slide film, I sometimes set the ISO 1/3 of a stop faster (say 64 instead of 50) to prevent blown highlights. So yes, there are multiple correct approaches; which are best for you is determined by your own results.