Support will tell you it's bad luck, even if it isn't, making the same assumption as me. I really do think it is just bad luck. You just tend to notice it more on grave robber since luck drops are the only thing it's good for, just like I feel like I find nothing but crappy items when I'm doing a low level job for crafting or quest products, instead of finding the products I'm looking for.
On the world where I'm currently key hunting, I've found 12 items in 49 hours, which is 24.49%, which is a little low (wearing the full holiday set), but not horribly so. My ring drops are ridiculous though, with 10 in 49 hours, which is 5.1% per 15 mins, when it should be 2% (premium adventurer/income).
Edit: You aren't the first person to make this observation, but like I said, I really think it's just perception. I could be wrong though. It also shouldn't be possible for the drop rates on salmon and trout to be wrong on river fishing, because the number displayed and the actual value SHOULD come from the same place, but I DO find more trout than salmon.