Why can I use 2 different private keys to connect to my SSH server storing only 1 public key?

On my server ~/.ssh/authorized_keys contains only 1 public key. However, I can connect to it using 2 different private keys.

One private key looks like this:

—–BEGIN RSA PRIVATE KEY—–
MIIEoQIBAAKCAQEAqaO1eFD6XTTbmcItrptJxRJr89oW2gwlFU0tt8oF/6ZbOfV9

p1RjkISOfKl7YEElZEBHsl/ikCgv2C8DIOTTknXYDXeIxi/PMg==
—–END RSA PRIVATE KEY—–

The other private key looks like this:

—–BEGIN OPENSSH PRIVATE KEY—–
b3BlbnNzaC1rZXktdjEAAAAACmFlczI1Ni1jYmMAAAAGYmNyeXB0AAAAGAAAABDwMB0nEv

LlI3s=
—–END OPENSSH PRIVATE KEY—–

Why?