How do you know the 17.3 wasn't made up by more sales of Uc2? Yes at that point Uc3 would be selling the most in the series but that doesn't mean Uc2 would cease to sell either.
Uncharted 2 sold 4.9 million units in two whole years (the time between UC2's launch & UC3's launch), you're telling me that Uncharted 2 increased it's lifetime sales by 50% in the five months period of UC3's launch window? That makes no sense, even Gran Turismo doesn't have insane legs like that.
Hell even Grand Theft Auto on PS3/360 sold only 2 million units in a similar timeframe (it sold 32 million by the end of 2013, 34 million by mid-2014).
The vast majority of those sales were Uncharted 3, even if you're generous & give 1 million of those sales to Uncharted 2 (making it have better legs than Gran freakin' Turismo), that still means Uncharted 3 sold 8 million units.
9 million is not only vastly better than any franchise some people mentioned here (minus CoD, BF, & AC), but its better than any Halo game except for Halo 3 (we don't have a definitive number for Halo 4 except that it sold less than Halo 3 which sold 10-11 million, so we don't actually know whether Halo 4 sold better than Uncharted 3 or not).
If Uncharted 4 sees the same growth Uncharted 3 had, it could sell ~12 million units.