The bot balancing is trying to add the correct number of bots to each team, but it does not count spectating bots since they have not yet joined a team, and because of this they are not counted during the bot balancing.
I should look into this. What bot balancing mode are you set to again?
You can try this test version which stops the bot balancer from adding more bots than the maxbot setting
mean6-00-02.zip
The maxbot setting is set here:
m_autobalancebots on <minbots> <maxbots> <ratio>