@swathdiver amps can and do degrade over time. As others have said it's usually the caps, although these dudes have Nichicon caps in em which are pretty highly regarded. There is a possibility that the nav radio in your '13 has a higher baseline output voltage (think 5v preout on an old school Alpine HU vs 2v on a barebones Sony from walmart), or a different crossover frequency which results in less distortion as the speakers wouldn't be playing frequencies they can't handle, and thus will sound better.
I suppose there could be a difference between the 2012/2013 UQA amps. When I was doing research for my UQS upgrade project I definitely remember seeing different part numbers for the UQA amps. The only way that could be determined would be removing the amp, pulling it apart, and looking up the numbers on the amplifier chips to find their cutsheets, which will tell you what their output is per channel and at what voltage/THD.
Or, if the truck wasn't new to you off the showroom floor, someone could've upgraded the amp in the past.
Here's the guts of my UQA for reference.