Padding is defined in pixels, so it's a number. And padding happens depending on the defined position.
If you define "center", no padding happens at all, because center is center. You can't use padding to define an "offset" from the center. If you define "left", padding is used as a defined left offset, if you define "right", it's used as a defined right offset.
So if you define "left center" and 5, the watermark is positions vertical center, 5 pixels from the left.
Multiple padding values doesn't really make sense, if you position the watermark left, any right padding is useless. But indeed, if you position "top left", the padding top and left will be the same.
I think it would also be handy to be able to define "10 50" as a position, so hardcode the location, which at the moment isn't possible either.