tag.db.scan_tags different behavior

I have access to a FX9550 and I've been experimenting with the command tag.db.scan_tags(500,true)  I've noticed that sometimes when I issue this command I get results that look like this:




And other times I get results that look like:




I am not sure why I am getting the different behaviors but I'd like to understand how I can force the former because scanned antenna number is important to me.