The butt-dyno can be pretty deceiving... I have noticed this myself though at times.
Over the past couple winters in chicagoland being VERY cold besides this past winter I definitely noticed a difference. The car will add extra fuel due to the colder air being inhaled to keep things cooler in the combustion chamber, I noticed abysmal gas mileage in 2013 and 2014 winters.
Once the warmer weather rolls in the computer adjusts accordingly for fuel/air and might feel like it is making a touch more power. This is just my understanding of it.