Why does the plane normal flip on a curve?

Hello, quick question, can anyone explain why the ‘plane normal’ planes on a curve flip as the go around.
You can see them flip or rotate 180 as I adjust the seam. How can i keep the axis consistent? or correct this?

plane normal flips direction.gh (4.6 KB)

The full answer is not quick. You can align planes using Z vector.

If you had posted a file, I would have shown you.

i think i have just uploaded the example. is this ok?

just tried aligning planes with unit z, seems to work. Is this what you had in mind?
Thanks for your help, i am very new at this.
plane normal aligned unit z.gh (6.3 KB)