Why does the "Interpolate Line" tool not show a straight line anymore?

[MarkusFinke]

Hello!

I am not sure whether I'm in the right section here to ask this question.

I wanted to use the "Interpolate Line" tool from the 3D Analyst to create elevation profile. But for whatever reason the tool doesn't show a straight line anymore but a "stepped" one (attached screenshot).

Could anybody tell why or what I've unintentionally changed?

Thank you very much in advance!

Kindest regards,

Markus