This is no error, since you selected "step on part", which takes the draft angle of faces into account.
If you want equally spaced steps despite of the draft angle of the faces, you need to use "step on plane".
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There is proabably some point alont the passes where they are going across a steep area, so they are moved closer together to maintain the stepover along the steep wall. But since zig and zig-zag patterns have parallel passes, the passes bunch together closer than this on the near flat areas.