Well I can tell you how I see things based on browsers etc.
There are many variables to consider as different browsers and monitor sizes resolutions they will all give varying results. I do not think either a or b is wrong. They both span results meaning they will limit 3 to a row to render a block because of a restricted screen size. The alternative as stated is to do no span and all 5 print regardless of screen size. That is how Pro works and I doubt you would want it to do that.
Maybe set your block to 3 images regardless. Maybe rebuild your thumbnails to 100 pixels instead of having them 150 if you want to display 5 photos. That is probally the correct solution IMO.