Private network blocks and their Subnet Masks confusion.

Hello /r/networking

I would love to get some input on why exactly it seems that /24 is the default mask for a Class 3 even tho RFC 1918 clearly says its a /16 for the Class 3 networks and Class 2 is /12. Why is it that in this modern era it seems forgotten what the actual standard is? I now know that RFC 1918 defines completely diffrent numbers than i was taugth in the past but i would like to know why is the text of RFC 1918 ignored by so many instructors?

I personaly find it quite wierd that the real masks are being forgotten. Like a Class 2 nobody thinks can actualy carry 1048574 Hosts even tho thats the amount they can carry acording to RFC 1918 when you calc it. And nobody thinks that a Class 3 can carry 65k hosts even tho thats the real number.

Well when i say nobody i mean nobody that has not gone and gotten them selfs the information straigth from the source.

Maby its a more isolated example than i think that the masks defined in RFC 1918 are forgotten by a lot of users. Also i use the Class 1-3 instead of Category because i dont know why actualy. I know the wording in RFC 1918 is Category.

Please point out any errors in this post as i would love to learn about them.