This is how the based pay rates were negotiated and adjusted a long time ago, and that's how it works.
Except, of course, that while this held true all those years ago, the erosion of the based salary's worth with inflation (generally worse on bases than in HKG), means that the monthly salary is now less than market in most base areas even if it "includes" the 13th month... in other words on most bases each month you get a fairly meagre salary with no chance of a 13th month "windfall".
Not that I envy the HKG guys in the arguably worse position of having to depend on the company's "generosity" every year.
Point is, WE ALL DESERVE A (LONG OVERDUE)
SUBSTANTIAL PAYRISE.