I can explain that:
Tank steering works on the principle of two drives equally parallel & exerting equal forces.

Your right wheel is angled inward which means the robot pushing into a counter clockwise spin & being the dominant force, is drifting the robot right. Your left wheel skids.

When the robot reverses, the right wheel is angled outwards & is the weaker force of the wheels. Right wheel skids & the bot generally follows the left wheels direction.

If there's some handy and it fits, map your drive out on square/graph paper. It'll help affix your drive down to a minimum of drift error.

You could use someone like me on your team, Matt!