Accelerometer`s equations

I have been going over Accelerometer`s tutorial and some aspects still confusing to me:

• values[0]: Acceleration minus Gx on the x-axis
• values[1]: Acceleration minus Gy on the y-axis
• values[2]: Acceleration minus Gz on the z-axis

The question is: when the device lies flat on a table in its default orientation
it outputs +9,81 instead of -9.81.

If the device lies flat on table Z-axis points down - so the Gz is *9,81*
therefore values[2] shall be ( 0 - Gz) = (0 - 9,81) = -9,81.

The question is Why is that ?
With X and Y axes there is no such confusion.