Because they are man made and variations happen between lots.

Did you confirm that all slabs were from the same lot?

Redoing the seam won't change the fact that the slabs are slightly different

You might want to add your question in your other post with photos, then delete this post.

This happened to me when my quartz counters were installed. There appeared to be a variation between adjacent pieces, but on closer look, one side had a very slight pitch which caused light to reflect on it differently, so it looked darker. We had it "fixed", but they did a terrible job repairing the seam, which previously had been almost invisible. So proceed with caution!

Please post a photo (in the comments works) - so we can commiserate with you! :)