Why does raycasting detect random stuff?

I have a part that's in a moving model and a raycast that detects the bottom. Sometimes it just detects random stuff, like the parts ontop or the parts on the side even tho i set it to detect stuff from the bottom, and when the part is in another part it detects literally nothing. How do I go around fixing this?

local rayOrigin = model.Detector.Position
local rayDirection = Vector3.new(0,-1,0)
local raycastParams = RaycastParams.new()
raycastParams.FilterDescendantsInstances = {model.Detector}
    raycastParams.FilterType = Enum.RaycastFilterType.Blacklist
    local raycastResult = game.Workspace:Raycast(rayOrigin, rayDirection, raycastParams)
if raycastResult then
    local hitPart = raycastResult.Instance
    print("something")
else if not raycastResult then
        print("nothing")
    end
end