How can the outcome of the route result be different than expected when optimizing?

This can differ due to multiple variables, but in general take into account: At cost optimization, the route generated from the API is the most cost-efficient route as requested by the client, which is not necessarily the shortest.
In these cases the result from the API is the most cost optimum option, despite being somewhat longer, taking into account the inserted fuel prices and daily cost and forecasted weather conditions. The fuel saved due to slightly better conditions outweighs the additional steaming time because the distance is longer.