It is due to difference in AF system between DiMAGE 5/7 and Maxxum/Dynax SLRs.
In general, the conventional film cameras (silver-halides) uses AF system called Phase Detection, while Video AF is widely used for DSCs.
The Phase Detection system, which is capable of obtaining defocus amount and achieves high speed AF operation, usually requires a large space between the optics and CCD to split through-the-lens light for distance calculation.
Thus, given priority for compactness, non-lens-replaceable DSCs are often equipped with Video AF system, which operates comparatively slower than the phase detection.