Season/promos/marketing don't factor in because the A/B versions are served simultaneously.
Visitors are randomly assigned one of two (or more!) versions. The recording of results starts and finishes for both versions at the same time so external factors are equal.
A/B testing is a method of comparing two versions of a webpage or app against each other to determine which one performs better. Learn all about it here!
www.optimizely.com
It's
not like they are showing one version in January and one in February and then comparing the two.
The test can glean all sorts of useful data and is very common for UX work