Could be that the receiver threads are not cut concentric to the aliagnment of the receiver,....or just the stock is slightly warped to one side a tad. No concerns if it isn’t touching or causing issues. You could always remove a little material from the stock on that side if it bothers you that much. And if you’re concerned about the barrel being canted to one side you could always verify by using a real long straight edge down the side of the receiver and measure the distance from the end of the bbl to the straight edge on both sides.
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