I've found this expression in the title of this game review: 'Kairobotica' Review - Kairosoft Throws A Fun Curveball
But what does it mean? The Cambridge Idioms Dictionary says that "to throw (somebody) a curve (ball)" means to surprise someone with something that is difficult or unpleasant to deal with, but this doesn't fit the context, does it?
"Curve ball" is often used in a difficult or unpleasant sense, but sometimes it simply means surprising. Take a look at the similar baseball-related phrase out of left field.
In this case, the reviewer simply means that the game is fun in a surprising, unique, or unexpected way.
It sounds like your dictionary is a bit incomplete regarding the usage of this idiom.
The article body suggests that Kairosoft did "throw [their users] a [metaphorical] curveball", by changing the "game mechanics". Because the resulting game was not what the author was used to from Kairosoft, the author had more fun playing the game than he expected. (Part of the fun of playing a game is overcoming unexpected difficulties.)
