The Ruby keyword not and the unary ! aren't the same. They have a different precedence, and therefore are not interchangeable, despite seeming very similar.
irb > not 3 == 4
=> true
irb > !3 == 4
=> false
The first example is evaluated as not (3 == 4), or not false, which is true. The second example is evaluated as (not 3) == 4, or false == 4, which is false.
Just use !. It's terser, it's less ambiguous, and it can be overridden.
Ruby's and and or have similar gotchas, which I plan to write about here in the future.