Horses and people shiver when they are cold to generate some heat. Older horses can no longer thermoregulate properly due to a lesser ability to digest what they eat and because frequently they can't chew what they eat properly. A lot of digestion begins in the mouth and if teeth are worn down or missing then the food isn't ground up and mixed with saliva properly to start the digestive process. Once the food reaches the part of the gut where it is digested and heat is generated in the process, the digestive process isn't as efficient as it was when they were younger. If they can't digest and thus produce heat with the process, they start to drop weight. Once they drop enough weight, then they can't thermoregulate properly anymore and need help, ie a blanket. You have to see the horse that is standing in front of you, not the horse the textbooks or old timers are always talking about. That may not be the horse you have.
There are all kinds of things that play into this too, the climate they're used to, their teeth, their age, their overall health and body condition, windy & cold vs wet & cold vs wet, cold & windy and so on. You have to evaluate each horse on their own merits, not just follow a general rule or do things one way just because "that's how we've always done them". A lot of research is done daily and new things are discovered daily.
My favorite example of this is: The little old person who is always cold. It's August, 95 F, humid and they are covered in Ben Gay and wearing a heavy sweater. They are no longer thermoregulating the way they used to.