My thinking...?

These are the sentences in question.
The source is Starting a Business While Raising Kids Alone

1. The whole thing has changed since I was in the workforce.
2. The whole thing has changed since I left the workforce.

I know both of these sentences mean the whole thing has changed after he or she left workforce.
And number 1 can't mean "Things started changing when I was in the workforce(it could started at any time during the period when I was in workforce) and is still changing even though I am not in the workforce now"

3. The whole thing has changed since I was in Seattle.
4. The whole thing has changed since I left the Seattle.

Likewise,
Both number 3 and 4 mean the whole thing has changed after he or she left workforce.
And number 3 can't mean "Things started changing when I was in Seattle(it could started at any time during the period when I was in Seattle) and is still changing even though I am not in Seattle now".

So I have a thinking: the reason why we interpret "The whole thing has changed since I was in Seattle/the workforce" in a different way than "The argument about whether homework is worthwhile has raged since I was at school." (which means "the argument started raging when I was at school) is probably that you can go to the same school only once in your lifetime, but you can join or leave the workforce many times(or you can visit Seattle many times). If the since clause refers to a event which can happen many times, then the since clause would mean "since sth stopped being the case". If since clause refer to an event which can happen only once, then the since clause would mean "since sth started being the case". Right?

Thank you in advance.