Hi !
I setup a hotspot on my rb750
when i went to ip / hotspot hosts i saw some of internal hosts mapped to the hotspot pool
but also a lot of public addresses were there !!
what are they doing there ? i should mention that their T/X was 0 but anyway i do not think they should be there
for example the 50.x.x.x address shown in picture below ! it is not even in My ISP Customers Address list even (50.x.x.x is not the public range of my country)
Oh !
found the problem.
My Bad !
The cable from ISP was not connected directly attached to the ethers of router board
it was connected to the switch and also a cable from routerboard ether1 was in the switch and a public IP was set on it.
and no rules in ISP side and in my router board, So many public addresses were appearing in the LAN (which is also connected to the switch)
But that is a good example of how the universal nat works! 
Yes 
I saw that ! every public IP coming in from that LAN port was assigned a static address in my hotspot range
I got what does universal nat do
but i still do not know how it is done, i mean client1 is into port1 of switch, client 2 into 2 and rb ether1 (lan) to port3
normally, traffic from cl1 to cl2 should never reach the port 3 (except in the beginning, arp query …)
so how can mikrotik jumps and intercept all these, mtik is my gateway, LAN to LAN should not reach the gateway, it is a unicast straightly from sender to destination port)