Issues with random samples in round robin lead assignment

In order to keep our lead database healthy, we built 2 programs in our instance: a lead wash and a lead attribution program. The lead wash program identifies leads who have had no activity in 120 days, and the attribution program reassigns them when they engage with our content or answer one of our phone calls. 

We're running into an issue where the attribution program is not assigning these leads evenly. We have 20 lead owners entered into the 'Change Owner' flow step, with 19 supposedly receiving 5% of the leads, and the 20th as the default choice who should also recieve 5%. 64 leads have gone through. Each of the choices received 2-3 leads, except for the default program, which received 11.

I found this thread where it looks like someone else was experiencing the same thing, but there's no other helpful info.

https://nation.marketo.com/thread/41705-round-robin-assignment-in-marketo

I attached some screenshots of our smart list and flow step. Can anyone help me figure out why these leads aren't distributing evenly?

Thanks!