They forgot to say something.
If the ending value not encounter the last sequence, the function return false.
In other words : 
Given a int $n, if ( $start + $n * $step ) < $end and  ( $start + ( $n + 1 ) * $step ) > $end, the function doen't give the maximum possible range, it returns a boolean.
So the last range number HAVE TO BE the end number.