Lots of reasons really, but they all have to do with how much work the CPU is doing. The faster the CPU runs, the more power it draws, the more power it draws, the more heat it generates. Launching a program makes the CPU rouse itself from slumber and do something, so it generates more heat. Gaming requires sustained effort so it remains higher than idle. Benchmark tests are designed to run the system as hard as it can and keep it there for a long time, so will generate the maximum amount of heat.
That said, your idle temps are on the better side of normal and your gaming temps are perfectly within reason. Your burn-in temps are getting towards the danger zone however. CPUs are generally only rated up to 100C, so you wouldn't be left with a lot of a buffer. However, since burn-in tests are intended to run all components as fast as they can go for sustained periods of time, if you're topping out in the 70-80 range, I wouldn't worry about it. If your gaming temps start creeping up past 60C, it's probably time to reapply some thermal grease.
Also, it might be worth doing a little research, but if you're using SpeedFan to measure your CPU temps, there's a better than average chance that you're getting inaccurate readings. There's no direct means of measuring a CPU's temp, so what motherboard makers do is stick a thermal sensor right underneath the CPU and estimate based on that. This was several years ago, but SpeedFan was known to be using a very outdated and inaccurate means of measuring CPU temp, which would cause it to frequently be 5-10C (or more) high. Maybe at some point the developer has replaced the old formula with a new one, I don't know, which is why I recommend doing a little research of your own.. If you're using SpeedFan to get the temps you reported.
Hey guys so I have the i7 4770k with a corsair h75 cooler on it, my temperatures are safe and I guess I feel comfortable where they sit at idle around 27c-28c and when gaming its around 46c-50c. Where my worry comes in is when I launch web browsers or some applications the cpu is sitting at around idle temps but then spikes up to around the lower 40c and back down...why?
ohhh and ps with prime95 I hit around 70-80c...good?