Production supply (parts, manufacture and distribution) & Advertising material (box printing, tv photoshoots etc) are where most leaks are generated.
If it isn't in production there would be very little leaks. The iPhone 5 was in production for approx. 3 months before it was launched, as was the iPad mini.
The iPad Original went to production the same week as launch.
I'm not convinced that they do many intentional leaks.
I was being more general about the intentional leaks. im sure some companies use this tactic.