If you choose to search from a stop, Travel planner assumes that you definitely want to travel from that stop. It does not indicate if there are better stops nearby, but provides you with a journey from your chosen stop.
If you search from an address or a location on the map instead, Travel planner looks up the nearest 100 stops, checks how far you are prepared to walk (2 km is the standard, but you can change this under “More options”) and then ignores stops that are too far away. It then looks up the best journey from the stops remaining. As it also includes the time it takes to walk to the first stop, you don't need to worry that you will always be given suggestions involving a 2 km walk. The nearest stop with the best transport connections for you will be selected.