Testers are technically development, so some of the "60 development staff" may have been from that department, but they tend to be very overworked (I'm not sure they would consider that to be the same thing as understaffed) so unless they realized there is no way they will have new products nearing completion for a good long time I find it hard to imagine this is testers.
I'm not sure that the Diablo team really uses contracted developers rather than proper employees. It doesn't feel like the Blizzard way of doing things where developers have a very low churn and company loyalty is well rewarded.
All guesses and speculation on my part. I was shocked to see this news because it isn't something the Blizzard I know would have done except as a last resort, and it's hard to imagine the company being near any "last resort" situations—the worst case scenario right now seems to be lower profit sharing for the higher ups, and I've never seen any of those guys as in it for the money rather live of the games. I know the company line at Blizzard is that Activision has no control or say in operations. I hope that's still the case, but this move, lacking any specific details, does smell like Activision all the way.
Patches for WoW – maybe – but expansions are huge effort!
Not to mention Blizzard aim for global simultaneous release for its games whenever it can. (Asia used to lag behind WoW's expansion up to a whole year compared to NA/Europe) Can't imagine the effort and coordination required for that task.
I'm saying that compared to the new projects(D3 and Titan), the overall personnel allocation is smaller for Heart of the Swarm and Mists of Pandaria.
The development effort on those isn't on a new codebase(unlike D3 or Titan). They're adding new features and reworking game systems in the expansions(especially with the WoW expansion, where they're completely rewriting basically every class), but the overall effort required to complete those isn't nearly as much as what they need for the new MMO.
In fact, when you've got a game as big as WoW, making sure that everything you add plays well with all the existing pieces is a mammoth undertaking - it would probably be easier if it was a new game! As you pointed out, these aren't simple 'Here's 4 new areas with some new enemy models expansions, these are major reworkings of large game features.
Although I've never worked on an expansion, from all the games I have worked on those kinds of things are definitely not simple.
And to be fair, an ideally managed company would actually have this happen every once in a while. A company like Blizz that has no problems hiring really should drop the bottom 10% of employees periodically and find replacements if needed. I doubt that's the real motivator here, but cutting 10% of employees isn't a bad thing at all as long as the people were picked properly.
Have you ever been a team manager where you'd get an email from the higher-ups saying "give me someone's head" on a regular basis?
Starcraft 2 and WoW are in more of a maintenance cycle with the expansions coming out this year. Neither probably need as many developers as the new projects at this point.
The layoffs are probably due to WoW losing subscriptions over the last few years, and Diablo 3 nearing completion. The majority of the 540 non-developer positions are probably level 1 support or low level IT related positions because they don't need as much support personnel. The developer positions are most likely contract employees for Diablo 3 that aren't getting their contracts renewed.