That is why Apple reports sales and these other manufacturers report shipped/sold to retailers.
Apple also reports shipments to retailers as sales.
Only the sales reported from its own stores are to end users.
Sometimes, but not for long. Once a product has been for sale for a month or so, then shipments pretty much match sales, because by then the retailers have gotten a good feel for throughput.
Technically, Apple also reports shipments. However, for the most part, Apple's shipments = sales.
Yep, eventually. We've all seen plenty of older devices sitting on retailer shelves at times, though.
They do carry inventory (# in channel), but they are very transparent about it and you can easily do the math to figure out the exact number of iPads they've sold for a particular quarter.
Actually, the quoted inventory channel numbers are not what's on retailer shelves, but rather what Apple has held in reserve to meet future requests.
(I.e. when Apple reports increasing to 6 - 8 weeks' worth of channel inventory, that's NOT what's sitting in all the retailer warehouses in the world. That would not only be insane, it would be illegal channel stuffing. It's Apple's job to keep inventory ready for old or new market sales.)
The reason Apple reports their channel inventory is because it's important to know if they're producing too much or too little for demand. (It also used to be a good clue as to when a new model was coming out, although that's less helpful now that Apple continues to sell older models as lower priced entries.)
The only time that their channel inventory report translates to likely extra end user sales, is when the number radically diminishes with no other possible explanation (like a new model coming out, or factory problems). At that point, it's assumed that end-user demand is extra high and thus so is retailer demand.