Question

Why can't you calculate the absolute magnitude of cepheid variables from apparent magnitude?

Answer 1

The first method is generic for any source of light but it doesn't consider any obstruction, modification on the way, while the second one is an instrinsic property of Cepheid variables. All in all the first method should be accompanied by corrections from other things like dust on the way, lensing, etc... Try observing if the discrepancy is systematic or random, if it is random it is very likely some correction as said, if not you might need to check for a deeper reason

Answer 2

We define the absolute magnitude of an object as the apparent magnitude one would measure if the object was viewed from a distance of 10 parsecs (10 pc, where 1 pc = 3.26 light).