Different owners of apps in /Applications depending on installation technique
If an app is digitally signed, then the owner will be assigned to the System. As this is a requirement for apps distributed through the Mac App Store, then the apps that you see from there will likely be owned by the system and not you.

With apps that you download directly from a vendor, then they may or may not digitally sign their apps, in which case upon installation, you may or may not be the owner. Digital signing provides different privileges to how the app can interact with parts of the OS, like getting beyond the firewall without an annoying popup to the user every time, so you may see even developer downloaded apps signed.

Be careful of taking ownerships of signed apps, as this can corrupt the digital signature and you may be force to reinstall the app.