The spell specifies that the player chooses the number and CR, but not the specific type. The interpretation comes from a post/Sage Advice/tweet/something from one of the designers, probably Crawford. Also, letting the player choose... is it sprites? Pixies? Whichever fey type has access to high level spells... leads to abuse, since you can effectively turn a CWB into a bunch of higher level spells by doing so.
I've found that having a random selection for the various conjure animals etc spells works well to prevent abuse, yet keeps them about as powerful as seems appropriate for their level. Conjure animals is the one I've seen used a bunch in my game, and sometimes it's the perfect creature for the occasion, while sometimes it's pretty far from perfect; but it's nearly always about as good as a third level spell should be. (Note that I don't, for instance, allow giant fish to be summoned out of water, nor do I allow non-water-dwellers to be summoned in the water.)