It has to do with how much positive participation the new user puts in, and how active the older user remains.
This are the criteria:
- quality of participation
- actual expertise
- ability to give clear and easy to understand answers
The criteria above came from my personal experience. I have seen many people that have joined various forums after me and have surpassed my by far.
If I don't spend a lot of hours on the forums, I'll have less chances to see questions of which I have expertise. Then when I actually see questions of which I have expertise I type an answer that is clear to me but isn't clear enough for the user to accept it. While I'm pondering for a way to make my answer clearer, someone with more experience/expertise and ability to give very clear and easy to follow answers will add an answer that becomes accepted and start getting many votes.
So, you can take the most skilled for person with the best ability to convey the best answer, but if he doesn't have time to contribute, he won't get the rating that some of that devotes more time will get. Also if a person that has less than the best expertise spend more time than anyone, but gives answers that doesn't work, they might inadvertently progress in a negative direction by getting votes for answers that are not helpful. The general public are not voting on the most answers. They are voting on answers that solves problems and are clear and easy to follow.
I'm finding that the more I participate, the better I'm getting at giving good answers. I'm also learning and gaining in expertise. But I still have a long way to go when you consider the 10's of thousands of users that are participating on the system.
I'm glad for the checks and balances to give reasons for a user to be careful and give productive answers, rather than just post anything to gain popularity.