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:

 

(tag_id=0x3000503130303030303200000000E19A,type=ISOC,first=2016-04-28T09:18:12.283,last=2016-04-28T09:18:12.741,antenna=1,repeat=30)

 

And other times I get results that look like:

 

(tag_id=0x3000201311248725010001000002B094)

 

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.

 

Thanks...