Some of the peculiar behavior arises due to C’s handling of expressions containing combinations of signed and unsigned quantities. When an operation is performed where one operand is signed and the other is unsigned, C implicitly casts the signed argument to unsigned and performs the operations assuming the numbers are nonnegative. As we will see, this convention makes little difference for standard arithmetic operations, but it leads to nonintuitive results for relational operators such as < and >.
Consider the comparison -1 < 0U. Since the second operand is unsigned, the first one is implicitly cast to unsigned, and hence the expression is unsigned, the first one is implicitly cast to unsigned, and hence the expression is equivalent to the comparison 4294967295U < 0U, which of course is false.