One theory I find interesting is the wage disparity between Mexico & the US. The way manufactured goods are assessed as being "produced" in one country or another is based on cost of production. So the folks working for very small salaries in Mexico do the production of the instrument, then ship it to the US and get it Inspected by a guy making $200k per year. This shifts the cost of production to the US side, making it a Made in America guitar. I'm told this is exactly the reason Fender moved it's production facilities near the US/Mexico border.
That's third party info that I can't verify, but it's interesting.
Other differences might be more concrete. I am no expert, but I hound websites looking for parts for my builds and I've noticed certain things like a bridge, for example, for a MIM would have steel chromed saddles while the MIA version would have stainless steel. The switch might have steel vs. brass contacts ...Things like that.
I'm glad you asked the question. I'll be interested to see more experienced opinions.
...so ever since then, I always hang on to the buckle.