Depends how you check your oil level when changing it as well though, i would imagine some people under fill their sump accidentally.
After putting new oil in the engine should be started for a minute then let it settle down shortly after, and when it stops rising in the window that is the level you want.
If you don't start the engine and do this procedure then chances are there's simply not enough oil in there, which will make the light flash on, probably intermittently.
I normally fill up to three quarters of the lower and upper level markings by the window, and never have to even top up between then and 4000 miles later, when i personally will change it again.
The recommended limit is 6000 miles, but it's just a habit i've got in to.