Thanks for replying.

Okay a few updates and details. I think I perhaps found out what was happening.

Machine 1. Not internet connected but blockchain updated in the last few weeks. Running bitcoin v21
Machine 2. Not internet connected blockchain now out of date by over a year or more ..(But blockchain was not even on day 1 and said 15 years behind when I wrote the first post).

Both have the same wallet.dat or if not the same wallet.dat exactly then an older copy of that same wallet.dat.
Because both show the same received tx on the gui front page. Slight different balance because a couple new tx have shown up on machine 1 because it has a more updated blockchain.  

Confusing things to me.

1. When I load this wallet.dat into machine 1. I see the same balance (pretty much)as machine 2 even now.
But although machine 1 shows in the transactions it has received to an address starting with a number 3 previously.
When I click on the tab in core that says window and go down to receive addresses.
That address 3 is not there I see different addresses.

When I look at this receiving addresses list on both machines then I see this.

Both machines match with the first address this starts with a 1
But then machine 1 does not have the receiving address starting with 3. Machine 2 shows this address.

Then when I was clicking generate more receiving addresses they were creating different addresses even after I noticed  a matching 1.
This matching address started with a bc1

Now that I've loaded a more updated blockchain to machine 2
And generated a lot more receive addresses I see they are all matching.

What I think happened was this. I'm just guessing.

I suspect that machine1 is removing addresses that starts with a 3 or bc1 as receiving address once it has some bitcoin sent to it.
So I had to keep making more and more until it got past those that machine 1 could see had recieved some tx but machine didnt know had received coins already.

The thing that seemed strange to me at the time was this.

Machine1 eventually produced a recieved address I could see that machine 2 had on there. But because machine 1 knows the next 2 addresses had already recieved coins ...then when I pressed generate more. It skipped those.  Where as machine 2 did not know this and produced them.

When they started producing the same addresses. Then when I went back I could see aha these addresses that machine 2 was generating had actually recieved coins on machine 1 in the past but those addresses machine 1 had taken out of the recieved address preview window.

I think there may be some settings somewhere where you can tell bitcoin core to show you all recieved addresses even if they had been used to recieve previously.

Strange thing is that it leave the address starting with 1 even though that has been used.

Maybe this is reasonable explanation?

Thanks.