Difficult to say for sure without actually seeing the tyres. A decent assessment of the overall tread depth across both tyres would provide a better picture. I still guess that both are worn in a similar pattern but one is overall more worn than t'other. Which side did that come off?
Also, if one side is low then you'd expect a smidge more inner shoulder wear that side as the camber will be effected slightly
If you think both are legal then I would go about trying to figure out any issues first. If not then swap tyres but make diagnosis a priority