There are a few issues that collaborated to give you the turn left instruction:
The ramps have no name - when this happens the next segment with a name along the route has that name propagated back in the directions therefore all the ramps will give a 'turn on Powell Ave' direction. Reference: Ramp naming
The ramp turn lanes are too shallow to generate a turn command, there will be 'stay' or 'exit' command. Reference: Junction Style Guide
and Turn instructions
The turn restrictions are not set where the ramps intersect either of the other roads. Reference: Turn restrictions
There is no way that you should have been given a command to turn off the overpass onto Powell Ave (there is no junction between the roads at that location) but the lack of names on the ramp may be generating confusing directions.
There's a lot of good information in the wiki but its sometimes hard to find. At least read through the Junction Style Guide
. That should help you understand a little better.