90mm or 92mm are just diff. ways of saying the same thing. Most case fans are designed to English (inches - with the basic design measurement spec being between the mounting hole centers on the diameter). Seldom does the measurement across the flats come out on the even 10mm, but that is used for naming convenience. For example: what are called 120mm are actually closer to 119 and 90mm are actually closer to 92 across the flats, but the distance between mounting hole centers is the constant as that is what really has to be accurate for case makers to do repeatable mounts. The distance across the flats and thickness can vary within some tolerance.
.bh.