I've labeled some things in your photo, that might help you understand what's going on.
The wires coming up into the panel are your service conductors, of which you have two "hot" and one "neutral". From there you have "hot" branch-circuit conductors, which are connected through circuit breakers. There's also a branch-circuit "neutral", for each branch circuit.
You seem to be concerned that one of the branch-circuit "neutral" conductors, is terminated under the same lug as the service "neutral". Which is a valid concern. You'd have to check the documentation or labeling on the panel, to determine how many conductors can be terminated at that terminal. National Electrical Code limits each terminal to one conductor, but allows device manufacturers to override that as long as there is documentation stating such.
It looks to me like that circuit predates the other two, and possibly even the panel itself. I'm guessing when they updated the panel, that conductor wouldn't physically reach the "neutral" bus. Instead of splicing the wire, they simply terminated it with the service "neutral".