The difference was in both Saul's case and David's case, they were confronted by a prophet of God. David immediately owned up to it. Saul dodged and squired and tried to find any and every avenue not to accept accountability. That's the difference.
Any doctrine that doesnt fit OSAS becomes a defense of "well he/she wasnt really a believer to start off with" depite dozens of scriptures talking about believers falling away. Its a default protective mode to not accept the truth of this matter.
You didnt answer the question I posed to your original analogy: Here it is again:
OK, working with your analogy, you said both get convicted, both accept Christ, both leave thinking they are saved. Then you proceed into a sanctification role for one of the believers. So here is my question. When did this believer (pursuing sanctification) receive JUSTIFICATION? During the process? Or at the start?
If at the start, then so too should the other believer have been justified when he called upon Christ. Not so?