I'm 100% certain it has to do with the fact that Tapastic is herding their traffic to the app, which doesn't utilize ads (not in the same way the desktop site does), which is causing less ad traffic to the desktop site, thus lowering the CPM, and bringing in less per ad impression.
I know Tapastic is working on that new video ad thing though that lets readers gain coins to spend on creators, but it's obviously not set up that way for everyone and creators will only earn those coins/keys if 1. they've got locked content to spend it on, and 2. they've got a fanbase who's willing to spend it on them.