Might be different thermal interface materials on different motherboards. Might even be different materials on different parts of the same boards.
I don't know ASUS thermal pads, but their thermal paste is some kind of generic industrial-grade bulk stuff. Some people automatically disdain it and do a complete repaste with whatever leet stuff they prefer, some of these report substantial (even implausible) performance gains and some report nothing but problems and nightmares. My personal experience across maybe a dozen ASUS mobos and ASUS GPU cards (but no ASUS laptops) is that their paste does a good enough job and keeps doing a good enough job for several years - those I've repasted all used some sort of sticky pink goop. I should point out that I'm not an aggressive overclocker, I do okay but I prefer "rock solid" machines which can run unattended, uninterrupted, overclocked hot and heavy punishment for days nonstop ... with minimum downtime ... so I prefer robust TIMs which can handle the abuse without cooking off, even if they don't perform as magnificently as others.
I expect that ASUS is more concerned with consistency than with cost-cutting, they'll choose TIMs which fit their spec (whatever that is) and fit their tooling and are proven to last across the entire warranty period with minimal flaws or faults or deviations, that's all.
(TIMs are big business, and premium/elite TIM brands are fiercely competitive business. There's lots of review, testing, and comparison sites online - although, as always, it's very difficult to find any which are truly unbiased.
I and others have written/argued much about TIMs in these forums before, lol, but the short version is that any premium-brand TIM is better than any El Cheapo TIM and the differences in measured temps between top performers seems to basically amount to less than one or two degrees, at most. Enthusiasts tend to champion their favourite TIMs quite zealously.)
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others." - Douglas Adams
[/Korth]