Thanks for that explanation, and especially the comparison screenshots. I finally understand what folks have been talking about all these years.
One of the cool things about PDFs is that the text and imagery can be vector-encoded rather than bitmap-encoded, which means the lines and curves are definited by mathematical formulas, not smal dots. That allows the PDF document to support "infinite scaling" without a loss of quality, which is something that bitmapped documents can't do--eventually the bitmapped dots get big enough to see and the illusion of smoothness breaks down.
My guess (maybe Brolin or Walter or someone can tell us the definitive answer) is that this is an instance of iFlyGPS being "held back" by the limitations of the WinCE-based iFly hardware devices, and those limitations being passed on to the iOS, Android, and Windows ports of iFly. I'd bet that the WinCE devices didn't have a practical way to display vector-encoded PDF documents from within the iFlyGPS software, so the workaround was to generate a bitmapped image of the charts/plates whose resolution was "good enough". (It's also possible that those bitmapped files would compress better and result in smaller data downloads and device data storage requirements, too.)
If that explanation is correct, then I wonder if the new iFlyEFB software will use vector-format charts/plates, and so support zooming without loss of clarity? (That might be a problem, since it would require different data sets to be maintained for iFlyGPS use vs. iFlyEFB use.)