The Transport for London (TfL) Journey Planner will always display the fastest route available at the time the request was made. At some times of day when the timetables line up, different routes can be displayed.
In this instance, according to the timetables provided by the Bus Operators to TfL, the 55 to Mare St with a change onto the 388 is longer than route suggested by Journey Planner. This is also affected by the 149 running more frequently than the 388, as Journey Planner also factors in the maximum likely waiting times.
However, TfL is always looking at ways to improve Journey Planner, and I have asked them to explore the option for customers to select both ‘fewest changes’ and ‘no walking’ within their Journey Planner preferences.