Values for the Active Messages Count metrics are point-in-time values - meaning it checks for the active messages that is not yet consumed by the consumer at that specific instant of time.
If you look closely, there is a slight deviation from the incoming and outgoing messages at that point of time.
There are times the messages incoming rate is very slightly more than the message processing rate - right from connecting to Service bus to processing the message complete. Also, if the processing of the message had not happened correctly, it would still remain active until retried.