I think it's possible that the low-end Android tablets (e.g., the $99 generic ones, not the Nexus or high-end Samsungs) a) make up a huge majority of the number of units sold and b) are used in a very different way than the iPad is. For instance, I have plenty of friends who are Apple all the way except for cheap Android tablets that their kids use for Angry Birds or watching video -- and nothing else.
FULL DISCLOSURE: I work for Apple.
Interesting point about resale value. iPads resell for insane prices on eBay. A lot of what might get chalked up as unit sales of cheap tablets in the Apple ecosystem involve second-hand sales of older models.
Historically Android tablets have had support dropped much more quickly than iPads. I'm sure there are lots more Gingerbread, Honeycomb and Ice Cream Sandwich based tablets just sitting in a drawer somewhere replaced by shinier newer tablets running Jelly Bean than there are iPads which have a much higher "hand-me-down" or resell value even if the original owners move on to something newer.
Anecdotal:
I know a few people just now considering upgrading from their iPad gen 1, and not a single person who is still using a first gen Android tablet.