How to measure the doppler effect perpendicular to the movement in laser doppler anemometry

I’m trying to grasp the principle of Laser Doppler Anemometry. I like it as it is quite genius I’d say. However, what struggles me is the circumstance that the Doppler effect perpendicular to the movement direction shouldn’t take place, or?

Maybe there are different principles because this one here is living under the same term but it looks differently:

Rückwärtsstreuung means back scattering
Vorwärtsstreuung means forward scattering
Seitwärtsstreuung means side scattering

Here I can realize the meaning of the Doppler effect as you measure the back scattered frequency (and also forward scattered, whyever). But nevertheless, it is perpendicular and shouldn’t exist, or?

edit: Here it is nicely visualized. What I wonder now, is, how the scattering/interference works. From my pov scattering should be totally arbitrary but yet, there is quite a structure given and the "gap distances" represent the frequency. I mean I can apply and calculate it.. but I don’t get it.
Why is that interference so specific and why does the signal, generated by the scattering from the particle, look so unambiguous?