Helmets are a bloody minefield! Oval fit, round fit, half a dozen different things like Mips, Kineticore, etc
Then there are the different standards and 3 different bodies testing the bloomin' things and other bodies saying that hair and test dummy heads mounted on spines can reverse the helmet ratings etc
i have been trying to find the perfect helmet for quite a while. What i have found is one manufacturers medium can be very different to the next and online photos from helmet reviews are worth diddly squat.
Be prepared to try on alot of helmets if you have any particular requirements. If all you want is a bottle top then it is much easier.