"Strong" Verbs

I seldom see this used here in the USA, so perhaps I should explain its usage in English. An example may help: the past tense of "sit" is "sat". I really don't understand why Americans who would readily agree that a machine gun "spat" it's bullets would report that someone "spit" out something from their mouth. Why not "spat"? I would say the same thing about another word which rhymes with that example, but delicacy and the reader's imagination make this unnecessary. There is another form of the past tense in English in which American usage and that of the British differ: Brits would say, for example, that a new coat "fitted" them very well: American usage would be that it "fit" the wearer. Of course, this is just one of many linguistic differences. When I arrived in the USA with my late first wife, she really did crack up our hosts on our travels across the country when she asked the man of the house to "knock her up" in the morning, using a common British expression that simply meant to awaken her.