It looks like that is their actual inventory of drives. At that scale, when 2TB drives become the most economical to buy that does not mean one immediately dives in and replaces all the 1.5TB drives. You have to do a bit more math than that.

It's also hard to tell whether they're actively decommissioning their 1TB drives or if they've just on a growth curve and they've done significantly more business while 1.5TB and 2TB drives were the more economical choice, but sooner or later the cost/benefit curves do cross and you do want to start decommissioning the smaller drives, it's only a matter of time.

There's a big difference between "storage" and "processing". But it's true, Google stores plenty of data too.

and why it takes more 2TB drives than 1TB drives...

That's the drives they actually have.

2280 * 2 + 3166 * 1.5 + 1 * 749 = 10,058 TB.