Misquoting Metzger

I'm intrigued by the conclusion in Prof. Metzger's The Text of the New Testament: Its Transmission, Corruption, and Restoration, in which Metzger and Prof. Ehrman take pains to correct the impression that the text of the NT was transmitted haphazardly. They write,

Lest the foregoing examples of alterations should give the impression that scribes were altogether willful and capricious in transmitting ancient copies of the New Testament, it ought to be noted that other evidence points to the careful and painstaking work on the part of many faithful copyists…. Even in incidental details one observes the faithfulness of scribes…. These examples of dogged fidelity on the part of scribes could be multiplied and serve to counterbalance to some extent, the impression that this chapter may otherwise make upon the beginner in New Testament textual criticism (4th ed., 271).

One wonders how this conclusion can be squared with Ehrman's Misquoting Jesus. Indeed, the foregoing two paragraphs in their entirety would seem to serve to contradict the most basic impression found in Misquoting Jesus.