I don't think it has anything to do with the actual cost of digging. IMHO the primary reason is they don't want to keep digging for weeks. By baking in an artificial cost increase they make sure this will stop after a couple days maximum.
Yes, they seem to have set the digging period in advance. Somehow that seems dishonest to me. If they knew they were going to dig for 48 hours come what may, they should have said so.