They probably have a better hub/nut design with less threads that is quicker to cycle. Mechanics might have a different placement that let's the wheel coming off get out of the way more quickly. I don't think they jack the car up quite as high, so less time to get it back on the ground. It could also be down to risk vs reward. I think I remember Mercedes being more willing to have a slightly slower stop if it's less error prone. RBR could just be going for broke every time. What's more, they generally always nail it when it counts; their slip ups might be less publicized during the race, probably because they're happening on the second car (not in a conspiracy type of way). Williams had sub 2 second stops for a while. But that was before they started falling off on the car side.