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
which is missing the dip in the middle that bothered you.
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tye
(but my friends call me "Tye") |