The blue dot comes from your phone's GPS signal which gets displayed on the map as soon as your phone's GPS fetched a new location.
The marker for the bus on the other hand comes from the GPS tracker on board the bus. When that fetches a new lock on the bus location it sends that every few seconds to the bus operator's servers. Several seconds later it gets sent to our server, and again several seconds later it gets sent to your phone. This can add up to a minute or more.