All Android applications must be signed with a digital certificate for which you hold the private key. Because digital certificates are unique, they provide a simple way of uniquely identifying your app.
This makes them useful for tracking your application in systems such as Google Play Store, and for tracking your application's use of resources such as the Google Maps servers.
Written in Offical Document
Maps API keys are linked to specific certificate/package pairs, rather than to users or applications. You only need one key for each certificate, no matter how many users you have for an application. Applications that use the same certificate can use the same API key. However, the recommended practice is to sign each of your applications with a different certificate and get a different key for each one.
Each machine is having different debug certificates which is used to sign the build, debug.keystore
which you can check at: c:\users.android\debug.keystore.