I think the problem is the color of mortar, it would have blended better had they used a darker mix. Grout lines in stone are often irregular, though I do see a couple places where it looks like either the stone was chipped/irregular or they didn’t get it cleaned off well. I would suspect the latter.
How long ago was it set? It usually lightens as it dries, just wondering how dry it is as I see some places in the close up that look darker.