Cabin luggage issues

Isn't it confusing? Some airlines put it as 55 x 35 x 20 cm as the flight bunch that I am taking from KL to Istanbul (Air Asia, India Air, Oman Air) while for Ryan Air priority 55 x 40 x 20 (10 kg) + 35 x 20 x 20 (laptop bag). How do we buy a cabin bag nowadays? Go for the smaller dimensions always?