My resource start dates are different than my activity start dates.

This is an issue that routinely occurs. When building schedules, P6 likes to see the schedule built in sequence. Project>WBS>Activity>Resources. While I understand you can't always know the resources you will have on the activity, if you start the activity with no resources and then add resources, the resource will pick up the progressed activity date. P6 sees that the activity started with no resource, it has been added, but it thinks or suggests that resource can't work from the start of the activity because you just assigned them. Try adding your resources before you mark the activity started. If you absolutely can't, you can try changing the start/finish dates of the resource assignment.