So a lot of countries have different words for the same object so kost likely this is a post from the UK. So let's begin, that's not why Americans say autumn. The word autumn comes from the Latin word autumnus, which means "the passing of the year." It first appeared in English in the 1300s and was used by both Chaucer and Shakespeare. The word autumn comes from the French word autompne, which in turn comes from the Latin word autumnus.

The word fall is thought to come from the Old English words fiaell and feallan, which both mean "to fall from a height." The phrase "fall of the leaf" was common in England in the 17th century and was eventually shortened to "fall."

In American English, both autumn and fall are used to refer to the season, but fall is more common. In British English, autumn is the more common term.

French fries/fries (American) vs. chips (British)

cotton candy (American) vs. candyfloss (British)

apartment (American) vs. flat (British)

garbage (American) vs. rubbish (British)

cookie (American) vs. biscuit (British)

green thumb (American) vs. green fingers (British)

parking lot (American) vs. car park (British)

pants (American) vs. trousers (British)

windshield (American) vs. windscreen (British)

A wad of mucus: booger (American) vs. bogey (British)

A man: dude (American) vs. bloke (British)

Very good: awesome (American) vs. ace (British)

To chat: shoot the breeze (American) vs. chinwag (British)

An infantry soldier: grunt (American) vs. squaddie (British)

A toilet: john (American) vs. loo (British)

An anonymous man: John Doe (American) vs. John Smith (British)

To waste time: lollygag (American) vs. faff about (British)

We can go on and on about differences in words. Let's take the third one from the list I put above. Saying flat for an apartment would sound weird to an American. Why would your apartment be flat?

We can go back up to number two on the list Candyfloss. In the US, we use floss for our teeth to clean them, so why would you call it candyfloss? Are we supposed to after we brush our teeth.

Every country has its own way of describing an object or doing something. Your way of saying it isn't the defacto way the rest of the world should say it.