Not quite right mate.
The star rating system is not based on a players ability. Rather it is based on his ability compared to the rest of your team. For example, if you have a youth player at your team, it is likely he will only have around 1 star current ability. However, if he is loaned out to a lower league club, he will probably end up a 3-4 star because the standard at their club is a lot lower.
A minus potential ability means that a player will always have different potential ability every time you start a new game. SI cant always exactly guess just how good a young player will become, so this is why they do it