[Nathan]

Its also the culture and different vocabulary/phrases.

For example, the well known "apartment" vs. "flat", "flashlight" vs "torch", "buck" vs "quid"

You could get away with saying "I'm easy" in Australia and everyone knows you mean you're easy going. The sexual connotation doesn't exist like it does in American/Canadian English.

When giving instructions, checking for feedback with "okay" vs "yeah?"

There are many small differences that add up quickly.