The Post Grid Gap setting in the Post element creates a padding-right on the last row as well and thereby messes up the centricity of itself. Nobody would want that.
For curiosity (and I always like to learn new things), I would be keen to know why padding-right and padding-bottom are used to create the gaps instead of the grid-gap property which would have resolved this issue in the first place.
But at the end of the day the main reason I post this is as an improvement request: It would be just nice to remove that spacing at the last row.