['Not was delivered email' IS] versus ['Was delivered email' IS NOT]
What is the difference between the two?
I'm making a simple (engagement) program with a smart list flow like this:

I would expect that if the recipient has not previously received "Test Email C", the flow would send Test Email C, and if the recipient HAS received Test Email C before, the flow would instead send "Test Email C-x".
However when testing this ('run once' batch campaign), the flow is sending email C-x instead! And when running the batch campaign again, it sends C!
Then I make a completely separate (engagement) program with a smart list flow like this:

In this program the flow sends email Cx2 first, and then Cx3 when I run the batch again. This is the desired behavior but I don't get the logic differences behind the two setups.
I'm lacking a general understanding of the "not" logic I guess!