Is it possible to get accelerometer magnitude value lesser than 1G?
If you are looking at individual samples or short term averages then that's perfectly reasonable.

MEMS accelerometers have 3 important characteristics, a bias offset, a drift rate and a noise level. 

The bias offset is as you'd expect from the name, the average reading at the 0 point. The datasheet should be able to tell you the maximum this should be. What is a little odd is that the value can be different every time you power the sensor up.

The drift rate is how fast the bias will change over time. 

And the noise level is how much random noise is added to the current measurement, this is generally large enough that for any meaningful accuracy you are typically looking at having to average a few thousand measurements.

There is also a non-linearity in the output but this is normally fairly constant and compensated for at the factory. It's never perfect but it's normally relatively small in comparison to the other errors.

Each of these parameters is different for each axis.

So yes, getting a value under 1 g is quite possible. As is getting a value over 1 g. In fact at times it's impressive when you manage to get anything usable out of the results.