"Red" symbol indicating unread emails?

Very confused as all of a sudden? I see in my "Mail" icon the usual red circle with purportedly, unread emails. However, In the past several days? This number keeps GROWING, now up to 31 and every single time I check my email, there is only 1 or 2 or none. So I don't know WHY it's indicating the growing number in red circle when there is nothing there or one or two when I go to check?

Hm. GREAT mystery of the Yahoo mail system? Or what?

Thanks a gazillion for any enlightenment...