I believe you may have the model versions backwards.
See: http://m.gadgets.ndtv.com/asus-zenfone-3-max-zc520tl-3640-vs-asus-zenfone-pegasus-3-3583
I have a 2gb version and it has the "M" processor.
I think it's not about which RAM belongs to which version. It mainly has to do with processor versions, and both versions have 2gb and 3gb RAMs.
I have a 3gb RAM + 32gb ROM version, with the MT6737M processor.
Screenshot of firmware details attached