Anywhere that's not masked on the board will get solder. Anywhere.
Some IC's are electrically or thermally bonded to the board, e.g. MOSFETS and other power handling components, which is certainly what would have gone in this spot. The part where the MOSFET's heat spreader would touch the board is not masked, ergo it gets solder on it. Your other missing-chip spots definitely have solder on their connecting pads, where the pins for those chip(s) would go.
That's just how it works. This is not unique to ASUS or any other manufacturer.