Why does the Play Store's download percentage never reach 100%? I've owned two Android phones so far. While both are HTC ones, I've used several customised ROMs on the older one, so the following does not seem to be specific to one maker or installation.

I've noticed that whenever I download an application from Google Play Store, the percentage never reaches 100%. It's usually between 40% and 60% when the progress bar disappears and the status changes to "installing".

I considered the possibility that the system begins to install the package before it's fully downloaded but, besides that being a bad idea, many times the installation takes a much shorter time than the rest of the download would have (assuming constant speed).

It seems that, instead, the percentage reported is wrong, and that the file is in fact fully downloaded while the screen shows 40%. Does that happen to others as well? How is it explained?
This started happening at the same time that Google introduced the concept of "delta" updates.  This means that only the portion of the app that has been modified is sent over, as opposed to the entire app.  However, they never changed how the Play Store app calculates the download percentages - it's still taking app's full size as the base 100%, instead of the size of the delta update being downloaded.  It simply never bothers to check with the download manager (which works correctly, as reflected in the notification tray) for the correct size of the delta.

This is why you are seeing the percentage stop at random times - the difference in sizes of the full app and the delta is what's making the play store think the download is larger than it actually is.  For example, if an app's full size is 10MB, but the update is only 5MB in size, the Play Store will still think it's downloading the full 10MB, and will stop at 50%, when the 5MB download completes.