Hi @christammm,
I presume you are talking about an AB test? In this case visitors are not meant to change an experience. I suppose there may be some rare cases where it could happen, but they should be rare. If you are talking about an XT (experience targeting) activity then it is certainly possibly they can qualify for a new audience later and therefore qualify for a different higher priority experience.
As for Target reporting source compared to Analytics reporting source (A4T) for an AB test. There can be some cases with A4T where it appears like visitors are in multiple experiences especially for redirect offer experiences. These reporting nuances are unique to A4T and aren't relevant to Target reporting. So I don't think you'll find the same need to create an exclusion segment with Target reporting.