Why are the "M" and "N" keys wearing out on my keyboard?

I'm using a couple of laptops - one is nearly three years old and for some bizarre reason only the "M" and "N" keys are loosing the symbols printed on the key. I could understand if the vowels were wearing out.

Someone helpfully suggested that I probably type "M" for "Microsoft" pretty frequently but in that case surely "O" would be even worse.

I've heard of more and more people who are finding that they're wearing out laptop keyboards and power supplies as often they're powerful enough even though they're several years old.

Of course the increased use of virtual machines for testing purposes on laptops does tend to demand additional power but for general purpose use it seems that rarely is the latest and greatest hardware required.