It's more like 21% for the last 3 years combined and 18% for FY 2016. But, yes, the effective tax rate (of 25-26%) that often gets reported for Apple includes a significant amount of deferred tax liability that Apple hasn't actually paid yet.
In very broad strokes (I'm happy to discuss it in more detailed terms if you'd like), this is Apple's situation when it comes to paying income taxes:
Apple pays quite a high rate of U.S. income taxes if we look at the profits it makes from domestic sales - above the (federal) statutory rate of 35% even. That's because (1) it pays U.S. income taxes on a considerable portion of the profits it makes from sales in other parts of the Americas as the U.S. is were it considers the value to be created, (2) it pays a (relatively) small amount in state income taxes, and (3) it pays some U.S. income taxes on profits it makes from sales in other parts of the world as it remits those profits to the United States (i.e. to the parent company).
On the other hand, Apple pays quite a low rate of foreign income taxes if we look at the profits it makes from foreign sales. Most of those profits don't get remitted to the U.S. (i.e. to the parent company) so Apple doesn't pay U.S. income taxes on most of them. That isn't Apple shifting profits offshore as some companies do, those are profits that can legitimately be said to be made outside of the United States. Apple does, however, effectively shift where those foreign profits are accounted for (i.e. based on where the value is considered to be created) such that it avoids quite a bit of foreign taxation. Also, Apple has accounted for - but hasn't yet paid - U.S. income taxes on about half of those foreign profits which it hasn't yet remitted to the United States.
So... Apple doesn't really avoid paying taxes on legitimately domestic earnings, but it does avoid paying (a lot) of taxes on legitimately foreign earnings. Mixed together those dynamics mean that Apple pays an aggregate tax rate of around 20% (on all of its earnings) and accounts for taxes (i.e., as you refer to it, on paper) at an aggregate rate of around 25%.
Apple would make out pretty well if the U.S. lowered the corporate tax rate to 15%, even if that came with an elimination of corporate tax expenditures (i.e., among other things, Apple would have to pay U.S. income taxes on those foreign profits). Apple has advocated for just that, an elimination of corporate tax expenditures and a lowering of the corporate tax rate. And Apple has done that not necessarily based on the rate going down to 15%, but rather at something higher that might mean that Apple ended up paying a little more in taxes. Apple thinks simplified tax policies and the flexibility that would come with them would be worth the (tax) cost to it.