Flock misread license plates in 71% of the alerts it sent to police in one California town
(www.businessinsider.com)
I guess that they are implementing crappy cameras? Not sure about american plates, but in most jurisdictions licence plates are optimized for easy OCR:

Note how each letter is unique and can't be transformed with a sharpie into another. That also makes OCR easy. If you use a crappy dashcam, you will get problems regardless. Esp. in low light situations at higher speeds.
So I would guess that Flock uses cheap sensors and cheap lenses. Which kind of makes sense when they are putting out thousands of scanners.