This is due to the fact that the system doesn't check for the default email filters when splitting up the group into halves. Once the target group is randomized into 50% halves for the original message and the variant, then the system will run the default checks on the following parameters before the message is actually sent:
  • Accept email is UNCHECKED in their profile
  • Deceased is recorded in their profile
  • Prior email HARD BOUNCE is listed in their profile
  • Profile is INACTIVE
  • Other problems with their contact information (such as an invalid email address).
If the constituent meets any of the above parameters, they will automatically be filtered out. The first half of the target group may have less or more constituents who are filtered out based on these parameters which can cause the split to be imperfect. If you need to have a more even sample size for both the original message and the variant, you will need to clean up your target groups to remove any constituents who might meet those parameters.