No proportion in bar size of Histogram

[#1]

Hi,

1. I chose "Graph Builder" and then I entered a numeric variable into Y Axis and a categorical variable into X Axis.
2. I chose Histogram and the response scale was set to percent (histogram style was 'bar')
3. Then I noticed that bars of different groups that represent the same percent value are somehow different in their size. Below is the graph. As you can see, for example, the bar of age 40-45 presenting 16% is much smaller than the one of 60-70 presenting the same 16%. Is there a reason for that difference? How can I change it?  

[#2]

Move the column from X-axis to X-group and change your distribution from Count to Percent

Count + X-axis

Percent + X-group