Attachment 214006
On these test cuts, I varied the speed, gap, power setting and air flow, in an effort to understand factors influence the formation of the patterns.. and I can't figure it out. The interesting part to me is also how the effect is always to the right or downward.

There are times when it doesn't happen at all, other times it's very pronounced.

Attachment 214008

I thought it might be the direction of the air inlet but rotating it does not matter.