What makes an Android app re-installable?

Through Eclipse I can easily re-install (without having to uninstall the app first), a small app I am currently developing, any change any number of times.

When I export that app ("release mode") and place it on a website, it downloads and installs without any problem. (I have to check "Unknown sources" for allowing installation of non-Market apps first, of course).

If I uninstall this app, then download it again and install it again, it installs without any problem.

However, if I try to re-inst开发者_如何学运维all this app without uninstalling it first, then Android notifies that the installation failed.

My question is why? What makes an Android app re-installable? (i.e. without having to uninstall it first, thus losing its existing settings)

You cannot replace a signed application with an unsigned one.

You fail to install it because it has the same app version. If you recompile it and then download it, it will update, because the version will have changed.