In the case of Randall āmispronouncingā cretin, the American English pronunciation would be kree-tin (Mike), but in British English this would be kre-tuhn (Randallās). This would be an understandable mistake if Randallās only ever read the word, or heard someone else pronounce it that way.
We know that Randall is a smart character from both his skillset in MI and his success in MU. More specifically, Johnny makes a point of hounding Sulley for his grades, and then replaced Sulley with Randy. Randy, who was Mikeās study partner. Randy, who was accepted into MU in the first place. This *would* indicate Randy being particularly well read, despite what his insecurity would lead one to believe.
However, it could also be that Randall knew the word through it being used by someone else.
We also know based off of Randyās insecurity in contrast to Mike and Sulleyās blind confidence, that he does not come from a Scaring household. That he needs Mike to point out the way that his camouflage can be of use to him in Scaring, instead of an embarrassment. A prey animalās defense response instead being a predatorās method of attack. If he were experienced in the Scaring world, he wouldnāt have needed this pointer. Heās new.
Randall also doesnāt look like any other monster we see around MI or MU, and itās only in MAW that the similarly structured Mr. Crummyham is introduced.
British English is the more globally standardized form people are taught in other countries, as a result of a whole bunch of historical stuff we donāt need to get into. All this to say that Randall couldāve very well heard the word said by a parent or relative who learned English in one of the British circles. A world like cretin, very known for being an offensive insult.