The intended behavior is indeed that each session consumes a license spot, you can also log in more than once as a regular user and that will consume multiple license spots as well. The difference with regular users is that each webservice call is a session on its own to prevent keeping the connection open, and usually wouldn't be immediately followed by several more calls.
However as you see in your log the consumption is quickly dropped again because the consumption only lasts as long as the action does.
If you really must, an easy way to prevent this would be to not call the Mendix webservice multiple times in the same second.