Joints also appear darker in some places even after drying

I tiled a basement room and then grouted it. Now, even after several days, the color of the gray grout is much darker in some places than elsewhere. At first, I assumed that there was still moisture in it and that after drying properly everything would be uniformly colored. But somehow nothing is changing anymore - what could have gone wrong?

Is it an old building? Maybe moisture is coming from below in some places?

Was the joint compound properly mixed according to the processing instructions?

Could it be that the tile adhesive was pushed up at these spots?

This often depends on different amounts of water in the mixtures. Sometimes it settles after a few months.

Thank you for your contributions. I'll try to respond here.
- yes, it is an old building, moisture is mostly on the edges, towards the wall. However, the dark spots can also be found in the middle.
- the grout was mixed as well as possible according to the packaging instructions. However, I had to add water because it was way too stiff. But if that was the cause, it would have to be equally dark everywhere. I only mixed for the whole room twice, so it can't be due to different mixtures.
- tile adhesive was pressed up between the tiles in some places. I wanted to scrape it out at first, but after I realized how hard it was and that the edges of the tiles were already slightly damaged, I left everything as it was. My thought was, put grout on it and it won't be visible anymore.