HEK,
Well... much to my shock you obviously weren't joking. If you actually believe that disconnecting the A/C on defrost (which isn't used this time of the year of course so is 100% irrelevant to both our discussion and your measurements until next November or December anyway) has increased your mileage by 20%-25%... and if you actually believe that you've somehow made a secret discovery that has escaped the notice of automotive engineers and manufacturers who spend hundreds of millions of dollars annually- searching for every last 1/100 of mpg increase in fuel economy... then there's nothing I can say: You've left me speechless.
I'd suggest there are 100's of other contributing factors that are accounting for your change in mileage including engine break in, oil viscosity, warmer seasonal weather conditions, less engine warm-up, seasonal traffic conditions, warmer vehicle operating temperature, tire inflation, your wife's driving technique, prevailing winds, differing routes and cargo loads, number of passengers, etc., etc., etc. The list is almost endless.
The only thing I can say for absolute certain that has absolutely NO bearing on the change in your mpg is disconnecting the A/C on the defrost setting since the defrost setting is not being used at this time of year- probably hasn't been used in at least 60-90 days.
Either way... I'm glad your wife has suddenly found the key to exceeding EPA estimated mileage- whatever that key may be. That's a good thing. On that we can both agree. 27 mpg is 27 mpg- regardless of how she's getting it.
Steve