Why can't they do just 4K, the resolution is already close to that and 4K is an industry standard... also with M1 chips I'm sure there won't be any issues with the battery
I'd rather be able to see watch true 4K content on my display, not this sub 4K weird resolution...
Who cares really? As long as you are not sitting there with a lens, there is no noticeable difference. What you should care about is pixel density, not the resolution.
But to answer your question, well, it's fairly simple. Official 4K is 16:9 aspect ratio, while Apple is using the more professional 16:10 aspect ratio (with these new displays going for the even more professional 3:2).