why arithmetic for bytes different than for ints?

[Vlad Levenfeld via Digitalmars-d-learn]

Can anyone explain to me why this test passes? I have read 
through the Types page on the dlang website and can't find 
anything that would explain this. Is it a bug or am I doing it 
wrong?

	uint x = 0;
	(x += 1) %= 2;
	assert (x == 1);
	ubyte y = 0;
	(y += 1) %= 2;
	assert (y != 1);
	// though I expected y == 1

Here's a table of all the types I tried ++x %= 2, where x has 
type long (L), int (I), etc:
	L	I	S	B	uL	uI	uS	uB
	1	1	0	0	1	1	0	0
	0	0	0	0	0	0	0	0
	1	1	0	0	1	1	0	0
	0	0	0	0	0	0	0	0

It all seems very odd.