Email Notification amount is sporadic

[corebreach]

I have instant email notification enabled for all threads I post on, and I notice that sometimes I only get 1 email telling me that people have replied, and at other times I get several emails telling me people have replied. This is confusing because
"There may be other replies also, but you will not receive any more notifications until you visit the forum again."

But I haven't visited the thread again.