My interpretation of this is as follows:
"Since the last decade" means after the previous ten years (2004 to 2014), that is, you're talking about the period from Dec. 31, 2014 to Nov. 17, 2015.
"For the last decade" means during the last ten years (2005 to 2015), that is, you're talking about the period from Nov. 17, 2005 to Nov. 17, 2015.