Actually, I think you're right... it only updates the automatic recovery file every x number of minutes (you set in options)
Why would you want it to automatically save it every 5 minutes under the same name? I can't count the number of times I've been working on a complex document, reformatting columns and tables for the 100th time, only to majorly screw everything up. I simply close without saving and reload the last version I saved. If it automatically overwrote the previous version, including major screwups just before I was able to go back, I'd be seriously pissed.