If I think an app deserves three stars, then I should give it a three star review, even if it currently has four stars.
If I say "well, this deserves three stars, so I'll give it a one-star review so as to pull the average closer to three" then I'm aiming to manipulate its rating, rather than rating it.
Note: I'm only guessing at Google's thought process here at the distinction between reviews and manipulation, and guessing at their detection algorithm; I'm not myself suggesting or implying that one-star reviewers are always engaging in manipulation. Of course you could honestly think an app deserves one star. The difference is unknowable to an outside observer. Only you can really know whether you're being honest, but that won't stop Google from assuming the worst.
If I say "well, this deserves three stars, so I'll give it a one-star review so as to pull the average closer to three" then I'm aiming to manipulate its rating, rather than rating it.
Note: I'm only guessing at Google's thought process here at the distinction between reviews and manipulation, and guessing at their detection algorithm; I'm not myself suggesting or implying that one-star reviewers are always engaging in manipulation. Of course you could honestly think an app deserves one star. The difference is unknowable to an outside observer. Only you can really know whether you're being honest, but that won't stop Google from assuming the worst.