Why does an accelerometer have an offset error when at its +1G position when calculating tilt angle on single axis?
A real accelerometer is going to have some nonlinearity and some offset error.
Because of how flat the sin function is near \$\pi/2\$, the difference between sin(85°) and sin(90°) is only 0.4%, so a measurement of 1.00V and -0.996V will represent that amount of error.
It's not unusual for inexpensive accelerometers to have a couple % of uncalibrated linearity error and some m\$g\$ of 0\$g\$ offset (that will also vary with temperature).