Well, that whole construction doesn't really add up, does it?
Let's assume your whole list is 100 leads.
In the first step you send to 25.
In the second you ignore 25, then send to 25%*75, which would be 18.
In the third step you ignore 43, then send to 25%*57, which would be 14
In the fourth step you ignore 57, then send to 25%*43, which would be 10.
At the end of the flow you have sent to 67 people. 33 didn't receive the email.
If I'm not mistaken we had that topic before and some folks proposed to split the list beforehand and create four equal lists. Another option would be to do the math a little differently:
Step 1: Send to Random Sample 25
Step 2: Send to Random Sample 33 (1/3 of 75)
Step 3: Send to Random Sample 50 (1/2 of 50)
Step 4: Send to all