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
[download]

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

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