I am using docker toolbox 17.03 version, recently I upgraded to 17.06 version using docker-machine upgrade command, after upgrading I see the server version is upgraded but the client version is still the old one.
i had the same issue. my issue was I had leftover docker.exe and dockerd.exe in C:\Program Files\docker and that was in my environment PATH.
anytime i ran docker version it would show Client: version: 13 Server: Version 18
I backed up the C:\Program Files\docker\docker.exe and deleted from that directory. then running docker version immediately showed matching version 18 for client and server. looks like client and server version should stay synced.