Detector rails are not precise at all

Detector rails detect minecarts that don't run on them.
https://gyazo.com/1fbcc6bc31d122557a7a5a30dd8780db

What I expected to happen was...:
I expected the minecart to go straight.

What actually happened was...:
It went left, meaning it triggered the detector rail. But if it goes left, it can't trigger the rail. Quasi quantum physic.

Steps to Reproduce:
Shown in gif.