Regarding PHIVOLCS using Tagbilaran as a point of reference for their reports, it's called standards & consistency. I certainly have no idea where Clarin, Maribojoc, or Canmaya Diot are, so I have no idea where an aftershock is in relation to those locales. However, I do know where Tagbilaran City is, as I assume most people also do. Considering that, using the most commonly recognized city/town as point of reference makes a lot of sense.