I thought that it could be something like that at first too. But I checked those:
Luck - Difficulty (+150) + Gear (+73.4) + Fortune Lv25 (+62.5) + 0 Natures + Ascension (+200) = 485.9%
Gold Find - Difficulty (+150) + Gear (+116.2) + Fortune Lv25 (+62.5) + 1 Natures (+25) + Ascension (+200) = 553.7%
Luck - Difficulty (+150) + Gear (+265.9) + Fortune Lv30 (+75) + 0 Natures = 490.9%
Gold Find - Difficulty (+150) + Gear (+286.6) + Fortune Lv30 (+75) + 4 Natures (+100) = 611.6%
Clearly both luck values don't average out to 638.4%. Although if you average them and then add 150 for the difficulty it is 638.4% but then surely this is counting the difficulty bonus twice?
I also noticed that my total gold find when using both is the same value as my wizard (main) alone. I tried equipping a different piece onto the warrior which had gold find (previous piece didn't) and the gold find stayed at 553.7%.
This is on floor 191, not a map or anything.
Could it just be an error in the stats page?