2003 Vulcan Nomad 1500Fi

I have been noticing something slightly odd in the performance of the bike I recently bought used, that I believed was my imagining until my son pointed it out one day leaving the gas station. When I fill the fuel tank, the engine runs smoother. As it empties it slowly and (I stress) only slightly begins running a bit more rough, the closer it gets to empty.

I purchased this bike a few months ago and the former owner kept this in mint condition. Any thoughts what could be causing this?