Moment Coefficient Variation with Angle of Attack
Its most likely from the AC. When it is said the moment does not change with AoA in AC it does not imply a completely straight line, it can vary a bit but is somewhat straight as yours also is around 0.1. And that 'rule' only goes for low speeds btw. Once AoA is increased above a certain point when separations effects come into play, it will no longer be 'straight' as your graph also shows around 8 degs.

If you think the line is not straight enough, maybe your AC is not calculated exactly (maybe it just uses the rule of thumb of 1/4 chord).