advice needed?

Dear Sir or Madam,

I could not locate an answer to my question in your database. If I miss it
please refer me to a right place.

Here is my question.
We had a contractors that installed marble floor in our house. at least 1/2
of the marble tiles that were installed is "uneven". from 1-4 millimeters.
Some of them is easy to notice simply by walking on the floor.
The contractors told us it is normal for marble tiles.

Thanks!