Confused about streetpass/voided villagers

Ok, so I thought I understood how voided villagers worked. My husband and I have been playing and visiting each other's towns regularly for a little while now. We've each picked up a couple of villagers from each other, but I expected them all and they were all recent move-outs.

The other night we tried streetpassing each other for the first time. I was at 10 villagers, he was at 9, but my void should have been clear because he had already gotten Antonio, my last villager to leave, from my void. The very next day, my husband said Nana's plot showed up in his town. Nana was one of my starters and moved out probably a year ago. I've moved out at least a dozen villagers since Nana left and I know I've visited towns with 10 villagers, which should have cleared my void. He talked to Nana today and, sure enough, she said she came from my town.

So is my understanding of the void all wrong? Is it different when it comes to streetpassing vs visiting another town? I just want to understand it better so I can plan around it because I really don't want a surprise villager the next time I drop below 10.

In short, visiting a town that's at 10 kills the void-stock isn't exactly true.
The system is like down below :

There are 2 individual storage to save the data of villagers who have moved out. ( Void-stock )

1. Void-stock for streetpass
2. Void-stock for actual transaction (visiting/inviting friends)

The message about streetpass
You can recognize the difference between when you just streetpassed and you got the voided villager by streetpass by the message of 3DS notifications.
When just streetpass : Message is like new model house of You-Know-Who from XX town.
When received the voided villager :

Void-stock for actual transaction

• Only one date is stocked.
• If your streetpass setting is off when the next day a villager is boxed up, this villager is for sure saved in void-stock for actual transaction.
• Only one data, so when villager A is in stock already and other villager B moves out, and if the streetpass setting is off, villager B is stocked.

Void-stock for streetpass

• Two datas are stocked.
• When ONLY villager A is in stock, and villager B moves out, B is for sure stocked.
• The latest moveout overwrites the previous data. So, when villager A is in void-stock for actual transaction and villager B is in void-stock for streetpass, and if villager C moves out and the streetpass setting is off, C is stocked in void-stock for streetpass.
• Also, when you delete the saved data in this stock by switching the streetpass on/off, this deletes in order by older ones.

The receiver side

• When you're at under 10 and have received villager A by streetpass, if you receive other voided villager B, then B will make a plot.
• If the B is someone who can't make a plot ( such as, a villager still in 16 cycling, or the one that's already in your town, etc. ) neither A nor B makes a plot.
• When you have data of villager A and B in void-stock for streetpass, and receive a villager C in void-stock for actual transaction, C will make a plot.
• But when the C is a villager who can't make a plot in your town, then B will make a plot.
• When you're at under 10 and have villager C in void-stock for actual transaction, and receive other voided vaillager D by transaction, C will make a plot.
• But when this C is a villager who can't make a plot in your town, then D will make a plot.
• You can receive only one stock per day in void-stock for actual transaction.

Hope this makes sense.

-* side note *-

These are verified before amiibo update. So I don't know if they added any changes after the update.

Thank you so much for the very detailed explanation! I didn't realize it was all so complicated or that there were two different void-stocks. I appreciate you taking the time to type it all out so thoroughly.

Ah felt at ease that it made sense, haha. You're welcome. Happy New Year!