The answer to my previous post is that the bridge is built on the Neapolitan. The Neapolitan is an altered predominant (altered ii chord), which, when in root position exists one half step higher than the tonic. Hence the half step higher in the bridge than the A sections in Db. Here's a link that explains the N6 chord pretty well. The art music approach of the common practice period usually features the Neapolitan in first inversion (hence the figured bass "6").