No coincidence, it's just not that common. You can add in Leonard/ LaLonde, Mayweather/ Maidana both fights and now Mayweather/ Pacquiao.

You have to have two champions from different divisions willing to face each other, with the higher weight holder willing to weigh under the lighter division limit. Normally a fighter wants every advantage possible so dual weight championship fights are rare.

Those examples were to show that even though Dawson was not at his championship weight his title was on the line.

Also the Light-heavy linage is pretty messy but you could make a case that it leads back to Ward.

Michalczewski-Gonzalez-Erdei-Grachev-Rodriguez-Ward.

I consider Ward to rightful Light-Heavyweight Champion.