There are several ways that a user who doesn't fit your test's target criteria can fall into your AB test.
If you have previously published versions of this AB test, make sure to check that they all have the same target as well. If you have a previously published version with a different target, and the target is sticky, then you will have users in your AB test who do not match your AB test's current target.
Users were forced into an AB test
In the user's tab, you can force a user into the control or variant of an AB test by clicking on the circle. A check mark will appear next to a user who is part of an AB test. Manually clicking this will force a user into an AB test, even if the user does not belong to the target. This feature is mainly used for testing purposes, to experience what the AB test is like in the application without having to match the specified target.
If a user is forced into a campaign or test, they will not exit automatically when they do not meet the target criteria. The user must be forced out of the campaign (by clicking on the variant of the campaign in the Users tab) to exit it. Once forced out, the user will then exit the campaign on their next session start.