Apparently now we're getting Generation Beta? And then presumably in 2039 Generation Chad?

This whole scheme is dumb, we should just refer to age cohorts by their decade of birth like in China.

Why are the number of years in each generation different?

Once upon a time, the cut off was mostly defined by major world events that shaped things. And different analyses use different dates. Roughly speaking:

Lost - Fighting in WWI, children in WWII
Greatest - Fighting in WWII
Silent Gen - Great Depression and WWII, post war adults
Boomers - Cold War
X - We saw the wall fall, the cold war end, and we mostly raised ourselves
Millennials - The first digital natives, first post-Cold War world view, global world 

The years are attempts to map experience onto a timeline that fuzzy on the edges. Latest generations have been defined much more statistically than based on shared experience of a generation, losing much of why the generations resonated up to a point. As an example of fuzzy edges, I know people born in 1980 and 1981 that are definitely Millennials and people born in 1983 and 1984 that are definitely Gen X.