The 20 extra points is based on one thing, I'm sure: The mechanics. They might have done some game testing and found that it was very difficult at the start to get an Aeris date based on their starting points, possibly due to there being more opportunities to hit on Tifa or do things with Tifa than there were with Aeris. The 20 points can't be used as any kind of evidence whatsoever, it's nothing more than what you pointed out it was: "Game mechanics". How many did Yuffie and/or Barret start with? 0 as well as Tifa? Does that mean that they are just as much a love interest of Cloud as Tifa is? No? Well, there you have it.

Ahaha, I also noticed the question evasion.