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?