Bad math as a marketing ploy. Simple stuff like saying a screen as a 21:9 aspect ratio instead of 7:3. Sounds bigger, right?
Well, my screen has 7936:5952 aspect ratio. It's called Hiper-HD suckas!
I would argue that this isn't a sinister marketing ploy but rather, it makes it easier to compare one monitor mentally to another.
16:9 is a fairly common aspect ratio, 16:10 is similar but a little taller. Comparing 8:5 to 16:9 isn't as simple as comparing 16:10 to 16:9. In the same way, it is easer for your brain to say "ah yes, 21:9 is wider than 16:9" than it would be to compare 7:3 to 16:9. If you're good at mental maths then yeah, it is easy to see that 7:3 would be wider than 16:9 but most people aren't going to want to do this calculation. Having the numbers in a similar format makes it far easier.