Zombie Horse Riders Spawn Error

Zombie horses with zombie riders are able to spawn despite light level preventing non horse riding zombies from spawning. This is occurring at night, in an area with a large amount of grass, and in an area with no other way for the horsemen to be entering due to incredibly high walls preventing them from jumping in. Keeping all of this in mind I suspect the following potential cause:

Zombie horses see grass blocks as a viable block to spawn on.

For whatever reason they see this rule as more important than the light levels of the area around them.

Zombie horses spawn on grass blocks regardless of light levels.

Because zombie horses spawn with zombie riders, the zombie riders also spawn without considering the light levels.

This results in otherwise safe areas no longer being safe for vulnerable mobs like villagers or players leading to much anguish when they are killed by the horseman in an otherwise safe area.