AOg, Cameron didn't promise a cut in migration by "tens of thousands", but to tens of thousands. Hence, the absolute net figure of people coming in has to be less than 100,000. Which 140,000 isn't. Doesn't matter about the size of the cut.
In that, then, Cameron has ailed. Doubly, because he hasn't even managed to cut net migration from non-EU countries to the required target. Having said that, I don't blame him exactly for this, it was always likely to be a missed target. The problem lies in making the pledge in the first place. Control over immigration shouldn't be set in terms of absolute numbers (and anyway, EU membership was always going to keep the absolute figure higher than any target), but in terms of relative gain. If immigrants are going to come to this country we should ensure that they are going to be beneficial to the economy, by taking on jobs or filling University places. As long as this is true, the current immigration levels could be sustainable even at a higher rate than tens of thousands. Applying an arbitrary cap in principle means turning people away who could bring useful skills etc, but are "over quota", which is the wrong approach -- setting a target for the sake of it.
Cameron's -- and the Conservative Party's -- mistake, then, is not in missing the target, but in setting it.