Magic is always a possibility

but no, its due to having users in different timezones.
The overall 'most' count is a single forum wide value, so you have a user in a different timezone who has seen 65 for today, and so set that count to 65.
Simple example - 30 registered 4 hours ago, 35 registered 2 hours ago. In your timezone, its 3 hours past midnight, so you only see the last 35 as today, the other 30 as yesterday. However, in Joe blogs timezone, its 5 hours past midnight, so he sees all 65 as today, and sets the max accordingly.
If you have a lot of users across many timezones, its better to use the 24 hour option, as everyone sees the same then.