Base Credit vs Estimated Credit vs Estimated PPD
Base Credit is what a WU is worth if you finish it and you don't have a passkey or haven't folded at least 10 work units with your passkey.
Estimated Credit is what you will get if you have a passkey and have folded at least 10 work units.
Estimated PPD = Points Per Day - this changes often depending on your wu you are folding at that time.