Originally Posted by GUWonder

Practices vs. published polices. The two are not entirely the same thing.

And when a more serious charge is not as likely to result in a conviction, prosecutors may pursue a less serious (or even somewhat otherwise unrelated) charge that is more likely to result in a conviction, as the purpose is sometimes as much or more about signaling or shutting down than equal justice for all under the law. The assignment for this kind of change in practice can be found in the same kind of thing that gave us the TSA.