Some of the drops you see in the NFL are truly mystifying - these guys train their whole lives up to that point and drop simple passes that just don't compute with how they could possibly do so.
As a Scottish person who played cricket his whole life ( along with a whole lot of other sports ), I became used to catching a very small rock hard ball that is hit at you at great speed in a whole host of differing angles and trajectories. To be honest, turning my hand to being a WR and training it briefly with the local Glasgow team, I found catching to be a fairly easy art to do well in comparison. The only difficulty is the field of vision with the helmet on and balls that are on a trajectory high above your eye line - you don't have the same ability to adjust your neck and look up as the helmet restricts this.
Basically, I see a lot of drips in the NFL that as you say ' my grandma could have caught that' and they are truly baffling.
As a Lions fan, and he is not a WR, Reggie Bush dropped a high number of particularly easy catches off of screens last season ( to go along with his fumbles ). Perhaps a change in coach and a firmer drilling of basics could stamp this out and increase his numbers more next season.
The entire Lions WR corps were dropping easy catches last year, including Calvin. It will be interesting to see if the whole team improves next year under different stewardship.