Why I have this situation on the hotspot?
Some IPs are assigned 1:1 but other are replaced with an "adiacent" IP.

Anyone?

Did you bind some IP?

Nope

If you look closely at your screenshot you will see that all of the ones marked "A" for authenticated have different "address" and "to-address", and the ones marked "H" for DHCP have the same address in both columns.

I am not 100% sure but I think that the reason is that you are using Universal Client both for unauthenticated clients (you set an IP pool under "/ip hotspot" for your server) as well as for authenticated clients (you set an IP pool under "/ip hotspot user profile"), and the IP pool you picked for both is the same pool that you also picked for DHCP on the interface running Hotspot. So 3 things are using the same pool of IPs. And when a Hotspot user authenticates, it feels compelled to pick a different IP for the user post-authentication, as if the pre-authentication IP is now reserved for capture-portal purposes and cannot be changed even after authentication to serve a different purpose.

I haven't figured out exactly why this is, but if you "/ip hotspot user profile set [find] address-pool=none", this will solve your problem, and Universal Client will continue to work even after the user has authenticated. This is how I have been setting up all of my hotspots.

-- Nathan

As usual a perfect answer.
Thanks a lot, Nathan.