So perhaps such wildly different imaginations could sometimes happen, and if so the GM can step in and correct the player's assumptions, but in my experience this is exceedingly rare. However, if this is a common occurrence then I would consider it a GMing mistake, and the GM should improve on how they disseminate the information.
If I described enough of a scene that you knew there was a halfling behind the bar, I'd likely give at least a brief description of the patrons and how crowded the place was. If I only described a halfling behind the bar? In most cases that would mean the bar is completely empty and there would be a reason.