Validation rules are back to front

From the docs:

less_than: Fails if field is greater than or equal to the parameter value or not numeric.
less_than_equal_to: Fails if field is greater than the parameter value or not numeric.

So, from your test, with a POST value of 1:
less_than[1]: This rule expects a value that is less than 1. The POST value IS NOT less than 1, so it will fail.
less_than_equal_to[1]: This rule expects a value less than OR equal to 1. The POST value IS EQUAL TO 1, so it will pass the validation.