Does their temperatures differ by about 15 deg C? Current readings could be wrong if they don't. If they do first slot is on top so card has worse cooling due to hot air from lower one. Back in 2011 or 2012 someone noticed that 59xx 7xxx cards took several a lot of watts more when they were hot (60 40 deg to 80deg). Apparently power leakage gets worse with smaller process node.
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