smallest interval between ratios in n-limit

3-limit:
4/3 : 3/2

5-limit:
6/5 : 5/4

7-limit:
7/5 : 10/7 *******

9-limit:
10/9 : 9/8

11-limit:
12/11 : 11/10

13-limit:
14/13 : 13/12

15-limit:
16/15 : 15/14

17-limit:
18/17 : 17/16

19-limit:
20/19 : 19/18

21-limit:
26/21 : 21/17 *******

23-limit:
24/23 : 23/22

25-limit:
26/25 : 25/24

27-limit
28/27 : 27/26

29-limit
29/21 : 40/29 *******

note how the 'pattern' is 'broken' at 7, 21, 29 . . . anyone got any explanations/comments?