There's another factor you don't see mentioned much. The Tories do not have their usual deep pockets and the spending threshold is much higher than in previous elections. They've sent out begging letters to members, a few big donors have said no more. More are likely to follow suit now you'd imagine. Hell Jeremy Hunt has even had to donate lots of money of his own to his own constituency. That's one angle to it but the other is that they need to spend money in seats they've never previously had to because previously safe seats are no longer safe.
It's the perfect storm, need to spend more, getting less.