It's also called "Series 7, Part 1" on iTunes as well.
I don't know if it's a similar situation to how we do things here in the US, but it could be like what happened on my show last year - we were picked up for a sixth season (13 episodes) fulfilling the original contracts that had been negotiated with our actors; in the middle of our production cycle, the network ordered an additional ten episodes, which we (internally and not on paper) referred to as "Season 7" because we had no idea when they would air.
As it turned out, the network only let five weeks go by in between these two batches of episodes, so it was genuinely a "Season 6, Part 1" and "Season 6, Part 2" for us... but my point is simply it could be a matter of actor contracts and salaries. In our case, we lobbed everything under the nomenclature of "Season 6" in order to keep all our actors and to pay them the same rates; had we officially (and on paper) called it "Season 7" we would have had to renegotiate with everyone in the cast and on top of that everyone on staff who would be returning would be entitled to a 3% bump in their paychecks.
But that's all on an American show. I have no idea how it works on BBC series, but it is something to consider -- with Gillan and Darvill leaving, it could just be that even though it's technically a "new" season (as in, series 8), they've for all intents and purposes been referring to it as and keeping it as "Series 7" so as not to have to renegotiate with Smith or any of the other production crew who might otherwise be entitled to pay raises.
Just my theorizing two cents