AB Testing in Smart Campaigns

Hello, 

I recently deployed an AB test through a smart campaign. The original recipient list was around ~410k people. Here is how I set up the AB test in marketo: 

Post deploying the AB test, I pulled a report -- and the campaign did split the sample size 50/50 -- however, the campaign only deployed to about 320k people total. I double-checked the smart list and the recipients should have been over 400k, and I also made sure there were no duplicates.

Does anyone have any suggestions as to where the gap could be in these numbers? Was it something I did wrong or am missing in the flow? Side note: I traditionally set AB tests up through email programs (which I generally think are easier) -- however, the email I deployed has dynamic content, and it is easier to set up subject line AB tests through smart campaigns. 

Hope this insight and background help & appreciate any additional tips on this issue, thanks!