Normally, you have a 3-Axis accelerometer measuring linear acceleration (surprise! ) in the three dimensions. A 6-axis accelerometer does not measure linear acceleration in six dimensions, that would be very hard. Instead the three "extra" accelerometers gives the angular acceleration which in the end means that you can measure acceleration in the 6 degrees of freedom a normal phone haves. I think the name 6-axis accelerometer is very confusing.
A gyroscope measures the angular SPEED, not acceleration, so combining a 3-axis gyroscope with a 3-axis accelerometer is not the same as a 6-axis accelerometer. However, the difference should be quite small for the end user.