Originally Posted by
cloudsail
This is psychological. When you are short of something you tend to pay more attention to whether or not it drops, and experience more disappointment when it doesn't. When you don't need something you may note with pleasant surprise when you see it drop, but care less when it doesn't, and because you aren't using it it builds up quickly in your inventory. Other than an increase after getting the first shard, for me mermaid scale droprate seems pretty constant. Some days I get unlucky and none drop at all, some days I get lucky and a bunch drop all in a row. The amount in my inventory goes up and down depending on how much I'm using them at any time. Same goes for any resource with a random droprate.