Thread: Adsense Estimated Unpaid Earnings

Can you tell me what is Adsense Estimated Unpaid Earnings?

It's been a some time since I loged in to my Adsense account and noticed under heading 'Earnings estimates'.
Today's estimated earnings £XXX
Estimated unpaid earnings £XXXX (unpaid earnings are a lot more than the payment freshold)
What is adsense estimated unpaid earnings?
My bank account details are set up but as far as I'm aware over the last few months the payment freshold has not been met.