I would assume that, as well as their 'live' storage of user data in their data centres full of hard drives, with whatever RAID type redundancy they use there, Google also has longer term archived backups which I again assume would probably be tape based. So, would the data from a dormant account actually be deleted? I can hardly see Google digging through their tape archives to delete cold-stored data from dead accounts. It would be so much hassle and be more costly than just leaving it as is.
It could very well be the same situation with actual 'live' data in data centre, belonging to dormant accounts.
Say you have a multi-TB hard drive in a data centre somewhere and, on that multi-TB drive, you have one or two users whose accounts are deemed to be now dormant. Would it really be worth Google's while to delete those one or two users' 'stuff'?
OK. the data savings would add up with thousands of inactive accounts, spread across data centres. But if all that freed up space is in relatively tiny amounts, dotted around thousands of hard drives, it would be a nightmare to manage and keep track of. Unless Google has some kind of system in place which is constantly shuffling data around to effectively 'de-frag' entire data centres?