I don't think they changed the C rate at all. I think the faster charge rate in mph can be attributed to a combination of less parasitic drain as @arg posited combined with the higher efficiency of the car. The Model S is rated around 285 Wh/Mi and the Model 3 around 237.
Let's assume for the simplicity lossless charging at 2.8 KW. In one hour:
Model S - (1 Hr * 2.8KW)/(.285 KWh/Mi) = 9.8 Mi of range added
Model 3 - (1 Hr * 2.8KW)/(.237 KWh/Mi) = 11.8 Mi of range added
Calculating the effective power from the losses
Model S - P = (7 Mi * .285 KWh/Mi)/ 1 Hr = 2 KW
Model 3 - P = (11 Mi * .237 KWH/Mi)/ 1 Hr = 2.6 KW
We also don't know how Tesla rounded the numbers. They may have rounded down the Model S and X numbers and rounded up the Model 3 numbers to make them look better. Tesla also doesn't make clear which version of the cars they are using for these calculations. The P100D is not as efficient as the 100D or the 75D (S or X). I'm going with the 237 WH/Mi leaked by Tesla for the Model 3, but I don't know if that is the small pack or big pack car. For AC charging the size of the pack isn't a big factor, but the driving efficiency is for the mph charging.
In any case, it looks like the Model S has about 800 W overhead when charging, but the Model 3 only 200 W. Due to rounding the two numbers are probably a bit closer, but they probably figured out how to charge with the Model 3 shutting down a number of unneeded systems that may be left running on the older cars. The Model 3 also has less electronics on board in general. The new battery management electronics might be contributing too.
I didn't run the numbers for all charging scenarios or the Model X, but that could be done fairly easily in a spreadsheet. If I didn't have to run off and take care of some chores that have been sitting in a few minutes, I'd slap one together. I might do so this evening if nobody beats me to it.
Let's assume for the simplicity lossless charging at 2.8 KW. In one hour:
Model S - (1 Hr * 2.8KW)/(.285 KWh/Mi) = 9.8 Mi of range added
Model 3 - (1 Hr * 2.8KW)/(.237 KWh/Mi) = 11.8 Mi of range added
Calculating the effective power from the losses
Model S - P = (7 Mi * .285 KWh/Mi)/ 1 Hr = 2 KW
Model 3 - P = (11 Mi * .237 KWH/Mi)/ 1 Hr = 2.6 KW
We also don't know how Tesla rounded the numbers. They may have rounded down the Model S and X numbers and rounded up the Model 3 numbers to make them look better. Tesla also doesn't make clear which version of the cars they are using for these calculations. The P100D is not as efficient as the 100D or the 75D (S or X). I'm going with the 237 WH/Mi leaked by Tesla for the Model 3, but I don't know if that is the small pack or big pack car. For AC charging the size of the pack isn't a big factor, but the driving efficiency is for the mph charging.
In any case, it looks like the Model S has about 800 W overhead when charging, but the Model 3 only 200 W. Due to rounding the two numbers are probably a bit closer, but they probably figured out how to charge with the Model 3 shutting down a number of unneeded systems that may be left running on the older cars. The Model 3 also has less electronics on board in general. The new battery management electronics might be contributing too.
I didn't run the numbers for all charging scenarios or the Model X, but that could be done fairly easily in a spreadsheet. If I didn't have to run off and take care of some chores that have been sitting in a few minutes, I'd slap one together. I might do so this evening if nobody beats me to it.