It is because the the 26-30 range is wider (covers more years) than the 23-25 range. If we divide each number of responses by the number of years in that range (skipping the top and bottom entries), then we get the following: 

 9-11 X   0.3
12-16 XXXXXXXXX   3.6
17-19 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX   15.3
20-22 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX-25.3-XXX
23-25 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX   18.3
26-30 XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX   14.8
31-35 XXXXXXXXXXXXXXXXXXX   7.4
36-40 XXXXXXXXXXXX   4.8
41-45 XXXX   1.6
46-50 XX   0.8
51-60 |   0.2

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which is missing the dip in the middle that bothered you.

        - tye (but my friends call me "Tye")

In reply to Bucket sizes (RE: My age is) by tye
in thread My age is by vroom

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