GM's eAssist: 25% improvement?

In a recent blog post, GM claims that their eAssist mild hybrid (starter/generator) system is good for an average 25% fuel economy improvement.

I'm not sure how they got this number, it doesn't pass the common sense test for me.

Here are the EPA ratings for the eAssist Malibu vs the 2.5L I4 gasoline Malibu, copied from the EPA's FuelEconomy.gov web site:

2014 Malibu eAssist

City: 25mpg

Highway: 36mpg
Combined: 29mpg

2014 Malibu 2.0L

City: 21mpg

Highway: 30mpg
Combined: 24mpg

So, comparing Malibu vs Malibu, by getting the eAssist system, you gain all of 5mpg for the EPA combined number, which is a 20% improvement, not 25%.

And these are EPA numbers!  In real life, where people will drive faster than the EPA does, the benefit of eAssist will drop further.