The average gym-rat just trains too much and too often to begin with, usually without eating properly. Take somebody that's been training with a 'volume' approach, then shift to him something like HIT, which uses low-volume, an emphasis on hard-work type training, and puts a premium on rest. This is where the HIT advocates are made, because inevitably they'll see rapid strength gains. Well, no kidding.
All the volume training has built up their work capacity, improving overall fitness, and generally laying a foundation. Now HIT comes in, takes away all the extra volume, and starts piling the weight on. The muscle which has never been allowed to rest and actually grow, is finally being given adequate rest time between workouts and being challenged with heavy weights.
This effect, where muscles respond strongly to sharp changes in volume, is pretty interesting. When you train a lot with higher volumes and higher frequency, then reduce both of those and really push the weights up, the body seems to respond favorably. This has been tested time and again in athletes. Actual performance and hormonal indicators back this up.
It's possible that some of the regulatory junk in the muscle fibers, like myostatin, gets switched on when you go through a ton of training. Myostatin, as you may or may not know, is a compound in the muscle that works to effects, while volume-based training tends to create more subdued but longer-lasting after-effects. When you combine different intensities and volumes, you produce different effects.