KWL

I have know nothing about readability tools. I want to understand how those tools calculate or estimate readability rates.
Its nature allows me to choose the best for my students because while in the text A readibility rate low, B has more rates. It is obvious it can be very good for students who are at lower levels or at upper levels of English.
But then I wonder how it does simply decide how readable a text. Could it be that it is a time saviour of a teacher because with one click you got your  results in front of you while sipping your coffee.

Text A

 Text B