Yes, if your pet outdamages you it gets to claim 50% of the XP.
To prevent this:-
Necros: Pull your pet off at 15% or so to let your dots tick down and ensure you've beaten Jobober (adjust accordingly if you think you need to). Effectively you're sitting medding as the mob is dying so you're not losing out in terms of time spent.
Mages: More commonly mages just let the pet do most of the work then suicide it as the mob begins to flee, and nuke to finish. Means carrying a lot of gems around but in terms of making sure you get the full amount it's worthwhile
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SKs: Same as mages
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