It has to do with the way Apple Apps are installed:
- In Windows: An application is made of lots of files that install in different places, including Executables, Resource files, DLLs, shared-DLLs, etc., all scatter all over your computer. Sometimes shared DLLs conflict are not compatible among different versions, causing issues that can become a pain to troubleshoot. These many files can be updated individually, which is great, as it makes the update file tiny.
I have ran many times into installed applications that show different versions in Control Panel and the app itself.
- In OS X: Applications are installed as one bundled package. For most applications, every component of the app is contained inside this package, and users don't have to worry about these components at all, saving a lot of time troubleshooting conflicts if these ever arise.
Updates are basically whole new installations of the application, but help preserve the integrity of the app package.
I rather wait for the file to download, than spend hours troubleshooting conflicts with a DLL that who knows where it is hiding.
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Apple will eventually do it, and do it right.