My guess is that Feynman didn't care much and basically flubbed the test on purpose or due to lack of motivation. That's why i've always been dubious of seeing this person or that person touted as the smartest in the world because of some IQ score: there are those who are highly motivated to score well and then there are others who are content to let their work testify to their genius. Feynman i'm sure was in the latter category and probably got a kick out of scoring only 125.
IQ tests by definition become wildly inaccurate when measuring the extreme upper end.
I once took two IQ tests in succession. 134 on the first (98.8 percentile) and 169 on the second (99.9997885357 percdentile) on the second.
The first IQ test was pretty standard, while the second involved the person administering the tests reading aloud a long involved essay on an obscure subject, then my taking a written test on the subject, which in this case was ancient Greek religion (cult and priesthood, etc.) as opposed to Greek mythology.
The problem was I’d just read a book on the subject. So I pretty much aced it and it wildly mismeasured my IQ.