olddefconfig vs. menuconfig (just open, save file and close)

Hi all, When you have an existing .config and just open `make menuconfig` say close and save the file you end up with a working (for compilation) .config file. As you do when you use make olddefconfig. In a recent (arm) project I have noticed that the .config files produced in the two cases differ (when starting from the same base .config). In general menuconfig adds more options (lines) than olddefconfig. What is the difference between the two cases? When should I use olddefconfig, when should I use the menuconfig approach? Thanks, -- Richard