How Many USB Controllers Are in the Late 2014 Mac Mini?
The 2011 Mac Mini only have USB2 ports. 
Since 2012, the Mac Minis have USB3 ports. 

That probably the reason. 

Also, there is only one physical USB controller the Mac Minis ref1,ref2, that never changed (note: On the 2014 Mac mini, it's integrated in the processor ref,ref)