std::is_sorted and strictly less comparison?

Question

I do not understand well the std::is_sorted algorithm and its default behaviour. If we look to cppreference, it says that by default std::is_sorted use

Answer 1

Per 25.4/5:

A sequence is sorted with respect to a comparator comp if for any iterator i pointing to the sequence and any non-negative integer n such that i + n is a valid iterator pointing to an element of the sequence, comp(*(i + n), *i) == false.

So, for

1 2 3 3 4 5

std::less()(*(i + n), *i) will return false for all n, while std::less_equal will return true for case 3 3.