From Wikipedia: This refers to Rabicano which is what your extra white hairs look like to me.
Roan, rabicano, and sabino
Main article: Roan (horse)
This true or "classic" roan has the characteristic darker head and legs.
A true roan is neither sabino nor gray, but instead refers to a pattern of evenly-interspersed white hairs on the body with minimal white hairs on the head and legs and few, if any white markings. The primary characteristics of sabino are white markings on the head and legs, often with roaning at the edges. When both roan and sabino are present in the same horse, it can be difficult to tell whether the roaning is due to sabino or true roan, especially if the white markings entirely cover up the telltale dark head and legs.
In some breed registries, the term "roan" is used to record sabinos, particularly with thoroughbreds and Arabians. Sabinos or sabinos with roaning have also been described as "roan" by Clydesdale, Shire, and Tennessee Walking Horse
The extensive roaning or ticking of this horse's barrel is typical of the rabicano pattern, which is sometimes confused with sabino. High white markings and patches are not controlled by rabicano. Rabicano
is another type of "roaning" or ticking characterized by scattered white hairs centered around the flanks, barrel and white hairs at the base of the tail. Only in the most extreme circumstances is rabicano linked to underlying pink skin. It is unknown if the roaning characteristic of some sabino-type patterns is due to the additional presence of rabicano or a separate mechanism. Rabicanos are also often identified as roans, even among breeds that do not have true roans, such as Thoroughbreds and Arabians.