A question on design? Bludgeoning, Piercing, Slashing

Hey Mark!

I've been reading through a lot of your monsters as inspiration for my own campaign, and comparing them to the worked guidelines from bagofholding https://www.blogofholding.com/?p=8469 and base book stats, I was curious if there was a reason why bludgeoning, piercing or slashing resistance like empyreans or similar have!

I was placing it onto a tier 4 enemy of mine but was curious about design, would love to hear back!