My guess is they made significant changes to the story to fit in with the AC universe, and they're changing the name to AC V.
Why do people think this makes sense?
Ubisoft's business model -- and this has been true for seven or eight years now -- is built around selling games of a very specific mold. They build franchises around core gameplay concepts that they judge to be both fundamentally appealing
and easily iterable: impressively rendered and distinct settings, open-world objective-based gameplay, and quasi-cinematic storytelling. They also leverage the generation transition as a key element of this strategy, by announcing a game at the tail end of a generation (when gamers are sick of their current options), relentlessly promoting it for years, and then launching the game during a window where gamers are unusually receptive to new IPs (i.e. a year to two years into a new generation.)
Ubisoft prototyped this on the PS2 with Prince of Persia, turning one well-received franchise return into four more franchise entries. They perfected this with Assassin's Creed, where the sheer force of marketing and the fundamentally appealing visual design made the series a perennial hit even though it took another two years to actually produce a good game. They've also backfitted this strategy to Far Cry and made it into its own repeatable, viable IP within the same general model.
The
entire point of Watch Dogs is to repeat this strategy and expand Ubi's selection of major IPs. If you break things into a 2x2 grid with "shooter vs. third-person action-adventure" on one side and "modern/urban vs. historical/exotic" on the other, they already have three of the four quadrants covered: Tom Clancy is the modern, urban shooter, Far Cry is the exotic-locale shooter, and Assassin's Creed is the historical action-adventure. Watch Dogs fills the last spot here and lets Ubi apply the same strategy for long-term franchise strength (content tourism and bar-filling, basically) to a game with a type of appeal they're not currently covering.
Assuming Ubisoft wants to rename the game and shoehorn it into the AC franchise is nonsensical. You have to start with the idea that Ubi planned and developed this game with, essentially, no plan, and that now the important thing for them is to make a little extra money on it by tacking on a proven brand. But that ignores the whole reason the game exists. Watch Dogs doesn't have a ludicrous budget to make money back on its own; it has a ludicrous budget so they can make a big splash, then sell you annual sequels for the rest of the generation. It got a gigantic marketing-blitz for the same reason: to drill the brand (and their ideas of what potential it could have) into people's brains well in advance of the release, and thereby guarantee it a running start.
Fundamentally, AC is an extremely fertile franchise (as years of annualized performance should prove) but it's still not an infinite well. Ubi can't sell people two ACs every year, which means their ability to grow the franchise or extract profit out of it is capped. What they
can do is develop other franchises that scratch similar itches, but are different enough to sell to an audience that only partially overlaps. That's why Watch Dogs exists and why it makes no sense to make it an AC game.
(This idea also feeds into the persistent fan nonsense that Ubi wants to do a modern AC game, when in fact the entire point of the AC franchise is to do historical-period tourism, and the modern element has been consistently reduced in importance since Patrice Desilets left.)