Why doesn't layered architecture translate easily to tiered architecture?

Question

Basically, I'm trying to understand why layered and tiered are that different, and why they don't translate easily to each other.

I understand that layered could be 3 separate class files for UI, BL and DAL. Whereas tiered can be a Project for the UI, which links to a BL project which links to a DAL like entity framework, and then a database. So why are/aren't they easily translatable?

Hope that makes sense, Thanks.