Crypto mechanics
The idea
Network fees compensate scarce transaction-processing capacity under the network's rules. Fee levels, finality and confirmation practices vary. A platform may impose its own withdrawal fee and waiting period. A transaction marked sent by a service is not necessarily finalized on the destination network.
A worked example
A low-fee transfer may take longer to be included during congestion. Increasing the amount sent does not always increase network computation proportionally.
Try it yourself
Trace the stages of a hypothetical withdrawal: service approval, broadcast, inclusion and destination credit.
A mistake to avoid
Promising a fixed arrival time without checking the network and service requirements.