An assumption is reasonable if it follows logically from facts we know about the world to some conclusion. For example, cars with more features usually cost more than cars with fewer. A washing machine with a detergent dispenser and 11 different wash modes costs more than one with 3 modes and no dispenser. A thermostat that just sets the temperature and does nothing else will almost always be cheaper than one with wifi connectivity and a companion iOS app.

Based on that knowledge, its reasonable to assume that a TV with more features will cost more than one with fewer. This assumption is wrong of course, but it wasn't unreasonable.

So that means an assumption is unreasonable if there's no reason you'd make it in the first place.