You are mistaken. Neither Federal nor PA law (nor, indeed, the laws of most states) dictates what is considered full and part time. Full time and part time are what the employer says they are.
You can try filing for UI for the missing five hours but I would not be holding my breath waiting for the checks to start rolling in. You also cannot apply for UI without your employer being aware of it; do you really want to go there for five hours a week?