I think a lot of the chinese whispers originated when the wheel was reinvented and POHs were replaced with AFMs which didn't have any of the power setting tables etc.
EG when I first came here I was told all sorts of "one size fits all" rubbish about RPM and MAP settings..(don't get me started on the oversquare bulldust!
) having flown Saratogas before coming to Aus I was astounded by this, then realised that the instructors really, really didn't know any better as they had no effective reference material other than "My instructor always told me blah blah" without any depth of thought..often just a thin veneer of knowledge that no amount of bling can make up for.
Similarly stuff about when to apply carb heat and turn off electric fuel pumps etc was "Over here we do it this way" and how the pommie way (as per the POH ) was all wrong and didn't apply in Australia.
This was decades ago but when I do AFRs I realise that nothing has changed.
The Day VFR Syllabus has it all there, but a lot of instructors seem to have never even seen it (student records often being a tick and flick exercise) let alone used it as a reference document. It doesn't seem to figure in Instructor courses, where it seems the only barrier to entry is financial wherewithal and not competency and ability to actually do the job.