It has nothing to do with the areas being different or similar. The point is that only one thing can be the first.
Mobile first, cloud first is a rehashing of spector if priority for their devices and services. They are saying when we build new versions of Windows or Office we will put the priority on mobile devices. When we build server software, the focus will first be on our cloud varieties.
Take for example Service Bus, software they make for Windows Server and their cloud offering. New features are added first to the cloud offering and later trickle down to the licensed on-premise version.