When drawing histogram without zooming, accumulated statistic is used to calculate mean - you see so to say “true” mean value. When you zoom histogram, mean value calculated based on bin content and bin position, which can differ from real values.
…and when you zoom back, the accumulated statistic is lost.
So the first value you see is the most accurate, based on the actual values inserted in the histogram, and the following values are approximations based on bin center positions.