Research has shown that while *some* Arabs have fewer than the 'normal' amount of ribs/vertebrae, others have the same number and at least one Arab (name escapes me at the moment) had more! Likewise, some non-Arab horses have been found with fewer than the 'normal' amount. So it looks as if -in reality - there is no 'normal' figure, just an average!
We don't need to count bones to know our horses are special and different