A vast inconsistency in reviews that have been published, despite not having enough characters. Why?

"Sorry, your review is too short. It needs to contain at least 600 characters."

This message appears while I write my review until enough characters are reached. 

Is this message something that only started in the last few months? 

I have seen any number of one-sentence reviews that are most definitely not 600 characters. 

How is it that these get published?

It's clear there are hardcore movie fans that will post thoughtful reviews and also people with brief opinions that will sometimes offer a single sentence, which, on occasion, they will copy enough times to reach the character count. 

It seems completely random and I cannot understand: is this 600-character rule applied to all reviews?