Until recently, each Google service had it's own distinct Google Storage and unused space couldn't be shared between services. As an example, you have 10GB of GMAIL storage but only 5GB for Google Drive. If you want to use 10GB for Drive and 5GB for GMAIL you couldn't.... until now.
The sultant of search has informed its citizens that it will henceforth consolidate all storage into one giant shareable pool of 15 GB. This means you can use any of that storage with any Google service you want (no more artificial storage walls or barriers.) If you decide to fork over some of your hard earned money for storage, it is now accessible from all services (in the past, extra storage wasn't accessible from Gmail.)
So go forth an rejoince in your newfound freedom citizen of Google-ville.
This consolidation has started to roll out but it may not have reached your account yet. Paid Google Apps account will see their storage bumped to 30GB.
Google blog post regarding the consolidation can be read here.