There are many factors at hand, including who manages you and who are your sponsors, among others - Bencic is from Switzerland and that's probably a factor too (a very rich country with many consumers of great liquidity). For example Venus Williams was give WC's for her first (ever) 4 events and they were all WTA's. Maybe Ana chose not to accept some eventhough she was offered - we can't know that (as I recall she was offered one for Strassbourg WTA and declined). With these restrictions it's silly to take WC's for strong events and bomb out in R1 if you can play more matches and earn more money some other way.
Remember that Bencic signed management contract with Octagon, who "owns" Luxembourg and Bastad (that's the way she got these WCs)