That would explain it.
Though it wouldn’t cause all this confusion if the dev initially added what you just said to their reply
(saying, that “It might take a couple of weeks before you’ll stop receiving mails after we unsubscribed you”).
Thank you for clarifying it.
Happy holidays. 
I never asked to be removed, so there couldn’t have been a delay between that and it “taking effect”.
What happened was I wrote a post about Avast spamming me and a Avast dev replied that I have just been removed! And then a while later I got spammed again. So what was the delay after being told that I was already removed?
The delay is not between you asking and them removing you.
The delay can be present AFTER they “hit the button to remove you” and the actual real stopping of emailing you.
In other words, it’s not always the immediate effect, even if they use their interface to remove you from the list.
It’s absolutely possible from the programming standpoint. Depending on how it’s implemented for every particular system (and company).
For example, there could be a some sort of queue / stack of pre-planned email sending for subscribed people in various lists, but the lists themselves are separate entities in a database.
It could be implemented the way they can remove you from the lists, but they can’t prevent already pre-planned stacks of emails for sending, which were automatically added to a queue, before they removed you from those lists.
It’s even possible the person who removed you didn’t realize that would happen, because people who develop the antivirus itself and those who developed the system for auto-emailing for subscribed people, could be really different people.
Just got another one.
So I suppose in a few years it will “take effect”?
And sure enough, I got another Avast spam email today!
These spammers won’t stop!
Can anyone explain how many tens of times one needs to unsubscribe before these spammers get the message?
I reported the email to Google, and will start doing it now every time my unsubscribe is disregarded.