...This is my 8th character in this game and always got the mace within 5-6 runs.
Sounds more like the normal spread. Eight times getting it pretty quick, one time it being as elusive as the Invisible Swamp Donkey is within expectations for a rare item drop.
Using 1.5% (for ease of math), 7 chests per run, 20 runs (for ease of math), than that's 140 attempts... so make that works out to 12% of not getting ANY to drop (binomial distribution). So 12% is roughly 1/8, so.... Eight runs you got it pretty quickly, one run you got to work for it. Not only is that within expectations, that's almost a bullseye.
See some nerdy discussion here:viewtopic.php?f=4&t=10279&hilit=binomial