When a new version of an application is installed over an existing version, instead of creating a new uninstall log file (unins???.dat), Setup will by default look for and append to an existing uninstall log file that belongs to the same application and is in the same directory. This way, when the application is uninstalled, changes made by all the different installations will be undone (starting with the most recent installation).
The uninstaller will use the messages from the most recent installation of the application. However, there is an exception: if an installation was built with an older version of Inno Setup that included an older version of the uninstaller than the existing one on the user's system, neither the existing uninstaller nor its messages will be replaced. In this case the uninstall log will still be appended to, though, since the file format is backward compatible.
The application name displayed in the uninstaller will be the same as the value of the [Setup] section directive AppName from the most recent installation, unless UpdateUninstallLogAppName is set to no.
The uninstall log-appending feature is new to Inno Setup 1.3. If you wish to disable it, set the [Setup] section directive UninstallLogMode.
Note: Setup can only append to uninstall log files that were created by an Inno Setup 1.3.1 (or later) installation.