The "Member Edited" trigger is fired when literally anything except when one of these values changes ( 'last_visit','last_activity', 'profilesync_lastsync' )
So yea, your observation is correct. It is also fired when somebody replies because his "total posts" count, last post timestamp, and probably also his achievement points value changed.
We thought that people would use Zapier Conditions once they need more control over the data, that said, we may improve this in the future and include a few filters into the "Member Edited" trigger.
Also, something to keep in mind, is that the webhooks aren't fired immediately.
We have a webhook queue, which collects the data and which is then fired via a separate request ( https://zapier.com/engineering/webhook-design/ leaving this link here for people who are interested in this)
Since you're on a demo that has probably not really that many visitors, the queue is also not running as often as it would on a live board!