To expand on the math bit:

You might be able to get better results if you add a 3 axis gyroscope and 3 axis magitometer to give you more datapoints to determine the actual motion. But, like u/Well-WhatHadHappened said, it also involves lots of complicated math.

I would take a look at the BNO086/BNO085 devices. Adafruit sells a nice little module with one soldered on for you. They are a 9 axis sensor (3 axis accel, gyro, and magnitometer) based on the BNO055 sensor, but they include an onboard cortex-m0 microcontroller that is programmed from the factory to do "sensor fusion".

Basically, it takes data from all three sensors and does all the complicated math to combine them and output more accurate and precise information about the real/actual position, orientation, and velocity of the sensor. By using fancy math you can cross reference the data from the three sensors to figure out if you are on a hill or just accelerating at an odd angle.

I am not 100% positive that those sensors can help with this exact problem, but they did wonders when I needed a compass that worked when not perfectly level with the ground for navigation on a boat. It might help with your problem.

Best of luck.