Edea received the power of a dying sorceress when she was very young. She was already a sorceress when she received Ultimecia's power.

You could say that the beginning and ending of this story happen at the same moment, when Ultimecia gives Edea her powers. Because that moment happens, the story starts and the story ends, so everything in the middle has already happened. If one of the things in the middle were to not happen, it wouldn't end correctly, meaning that it wouldn't start in the first place.

And there is a reason for time compression if you connect the dots. Compressing time would bring all time to a single point, which would bring all things to a single point, including all sorceresses. When the Great Hyne was defeated or left the world or whatever happened to him, he left a piece of himself in every sorceress. So, if all sorceresses were in the same place at the same time, one could kill them all, absorb all of their powers, and piece together the power of the Great Hyne once again.