I guess it is some kind of problem with the home-made counter (e.g., picking up electrical noise), or perhaps the counter is placed in a building with a higher background level.

As far as I’m aware, buildings made of blocks containing coal power plant slag have a higher background, but I don’t know how pronounced the effect is.

The normal background should be like 15–25 counts per minute for majority of Geiger tubes.

This is not my project, I just submitted it as an interesting website.

Perhaps if the discussion catches on, I can send the link to the website's owner.