It seems that team members are ranked based on their total credit and not by how much credit they actually contributed to the team. Since people who change teams do not take the credit with them, the rankings should be assigned based on how many credits were actually contributed to that team.

This has been discussed before and it's not possible, that information is not available.

... I just thought of a way how to do this, but it would be based on credit from the moment the feature is implemented, no historic data is available, so it will never be accurate. For example, I will most probably never leave my team, so all the credit I have build up in the past will not be part of the new "credit I made while a member of the team".

Also, currently I'm busy preparing to move to a new home so I don't have enough time to complete such a complex change and give it the needed attention after is has been completed, so it'll have to wait.

If the information on when a member joined a team is available, you could use the joined date to determine how much credit the member earned since they joined the team. This would allow for past credit to be taken into account in order to establish more accurate rankings.