A second problem is that I would have imagined they would report values something like "0..359" or maybe "-180..180" but instead they peak around "-150" and "+170". And the peaks aren't the same for each axis. So the numerical range isn't 360 but around 320. Yes, I can map these values to proper mathematical values, but it's a minor complication I could do without.
Yeah, its kinda confusing, and you are confused.
I suggest reading;http://en.wikipedia.org/wiki/Inertial_navigation_systemhttp://en.wikipedia.org/wiki/Accelerometerhttp://en.wikipedia.org/wiki/Gyroscope
The short answer - the accelerometers output is a measurement of 'g-force' (often expressed in units of ft/sec/sec) a measurement that is about how much change in some period of time. so you measure time change while in accelerating/decelerating to calculate orientation in 0-360 or -180 to 180. But it doesn't measure orientation. Gyroscopes do that.
edit/ ps: accelerometers, as you can see in the wiki, are SOOO MUCH more versatile than a gyro alone. detecting taps on a touch screen, pedometers, earthquake/vibration detection.... an accelerometer is a very useful device indeed - albeit confusing. (I think its the multiple axis' that contributed to my learning curve that Im still on.) /edit