500 Cars produced in 10 hours Achievement

I received the achievement in a 3k budget cars challenge which means I would have needed to produce 50 cars per hour for 10 hours, however, the metrics telling me how many cars I produced an hour never went above 35 cars.

And as I’ve just completed the 3000 budget cars scenario in just over 8 day’s and throughout the whole time I’ve been seen 35 cars an hour so I think the factory efficiency is not correct or something else is going on with car’s sold/in storage