TL;DR

CentOS installs on different hardware, but
1 Its all PCs which vary less than phones
2 A Ubuntu kernel, Debian kernel, and Elementary kernel are all different kernel sources.
As for Why they're different, see the above answers

CentOS installs on different hardware, but,

1. It's all PCs which vary less than phones,
2. A Ubuntu kernel, Debian kernel, and Elementary kernel are all different kernel sources.

As for your second point, see the answer(s) posted earlier.

TL;DR CentOS installs on different hardware, but 1 Its all PCs which vary less than phones 2 A Ubuntu kernel, Debian kernel, and Elementary kernel are all different kernel sources.

For Why on #2, see above

TL;DR

CentOS installs on different hardware, but 
1 Its all PCs which vary less than phones 
2 A Ubuntu kernel, Debian kernel, and Elementary kernel are all different kernel sources.
As for Why they're different, see the above answers