PV watts: standard module VS premium module

I have recently done some quick PV analysis using PV watts module in SAM (latest version) and found out the annual energy generation
difference between a standard module VS premium module is smaller than what I
would expect. See the attached screenshot.
in case you can not see it, below is the result

standard	1678kwh
premium	1684 kwh

This is the energy output for a 1kw system as a test, I have kept all the other inputs the same, the location is LA.
 

Looking at the documentation from PV watts, the standard module efficiency is 15% with a -0.47%/C temperature coefficient whereas the
premium module efficiency is 19%  with a temperature coefficient of power
of -0.35%/C. so I would expect the premium module would generate
0.19/0.15-1=27% more energy than the standard module. I have also checked the
web-based PV watts and it shows a similar result with SAM.
 
Anything I am missing? Why the premium module is marginally better. Any input is appreciated.
 

thanks so much!