It's based on how many items in the specific warehouse are shipping.amazon baffles me, i order some cheap headset for my daughter for school for $4, take the $1 no rush shipping option, and it ships immediately and will be delivered saturday. 2 things i ordered the day before using prime 2 day shipping wont be here til monday....
The no rush stuff shipped early because packers ran out of prime/expedited orders. The employees can't just sit and do nothing when those order types run out (unless orders are so low employees get sent home early, but that's not too often), so the no rush stuff which is lowest priority then get completed.
It's all just based on how many orders have been made and if no more prime orders, time to get the next down the line. Some warehouses have more orders than others, though.