https://www.postgresql.org/docs/15/functions-string.html, you can also use the site's search functionality.

It has to deal with the order of operations. The first one will only return true if both conditions are false (so the value has to be greater than 'm' and smaller than 'a'). The second one return true if the value is greater then 'm' or the value is less than 'a'.

As for solutions using [^a-m], they are using it as part of a regular expression (REGEX) to test if the characters in the evalutad string are not between a and m.

Hopefully this helps, I know I am a few months late.