I see your point, although the fact they're coming at the relationship from different directions doesn't matter as much as the fact that they got there does.
You caught the unspoken irony in that Betty seems to get shortchanged because she knew Paul when they were both kids. Now that Paul's 21, he has an appeal for Candy that he wouldn't have had if he was 15 himself.
But that's the way romances work. Timing matters. If you met the same person a year earlier or later than you did, neither one of you might have been interested. (And vice versa.) There's a huge element of pure chance in life.