Define what you mean by "tilt in the direction of the X arrows" - I would understand that to mean an imaginary normal to and above the PCB would travel in the direction of the arrow. In that case the X acceleration should go negative since the acceleration vector is now pointing away from the X arrow.
Test that you understand your axes correctly. With the Z arrow pointing upwards the Z accel should be positive, with the X arrow pointing upwards the X accel should be positive and if the Y arrow is pointing upwards the Y accel should be positive. The board may be mismarked, you may be reading the wrong registers, or whatever.
Then check that the same thing happens with the magnetometer (you have to establish the 3-D direction of the magnetic field of course).