Drives tend to run cooler when they are mounted in a case, despite the inside of the case being warmer than the room. This is because mounting the drive onto a metal chassis conducts heat away from it, and this is much more effective than the simple radiation of heat that you get otherwise. (This is why you have a heatsink and fan on your CPU rather than just a fan...)
I just bought a 7200.7, and like yours it was running at 47 degrees when it was hanging loose outside the case. It is now mounted inside and runs at 37-42, and this is with a mid-high-30s internal case temp (Shuttle XPC == v.difficult to keep cool)
So I'm not convinced that your test proves anything. To do a fair test you have to compare the drives in the same place, mounted (or not) the same way, and subtract the local ambient temperature to get a net heat output.