The $26K USA lot: Can't say with certainty without close inspection, but a combination of quantity and many unused 19th century stamps with OG (or the appearance of OG) is my guess. The difference in value from "no gum/regummed" to "part original gum" to "OG lightly hinged" is quite steep for most 19th century US, and the photos online tell us nothing about the gum condition; that's something inspection might reveal that changes the game.
The Italian colonies lot: I was a bidder on that one, too; there were some nice items, but so many of the used in the lot were favor cancelled (or had outright fake cancels) IMO that I wasn't willing to go very high for it. A collection of similar completion with mostly authentic postal cancels (and certification for the best items) would go for substantially more than the C$1,050 hammer; genuinely used Italian colonies are difficult, much more difficult than mint.
It may be the winning bidder is thinking not of adding to his (reference) collection, but of Internet resale. Many of the sets in that collection could be listed on eBay
tomorrow and sell for 10%-15%, perhaps more, of the used CV, favor/fake cancels notwithstanding. They're stamps most collectors don't see often but are eager to own.
The British Africa lot that went for C$4,200 had a similar situation: there were multiple 5/- and up values with fiscal cancels, or washed fiscal cancels. They look like very valuable stamps to inexperienced collectors and there are folks that take advantage of that and slap them on eBay
(or sell at a show bourse!) as 'used' or 'uncancelled/unused NG' and attract bids in the hundreds of dollars. The existence of these high values in the collection, even though they aren't quite copacetic, pushes up the hammer price.