Shoggy & Phatboy69: "some fans" with such problems are particular models or few particular bad fans of worth quality in batch? If there are several fans on one channel, maybe it's worth to change from which one rpm signal is fed?
2Phatboy69: again, if it's several fans on one channel, and you use some kind of Y wire splitter for interconnecting fans, are you really sure that tach signal wire is connected from only one of fans on that channel? I've heard before that with some DIY wire splitters where user that made them wrongly connected together yellow tach wires from all the fans on channel, then rpm proper reporting got fscked up aswell.