how does postfix determine hostnames in 'received' header?

[Michael]

hey,

i have multiple nullclients with the exact (!) same main.cf. all but one 
show the received header as expected, e.g.:

        Received: from host.domain.tld (host.domain.tld [10.0.0.1])
                [...]
        Received: by host.domain.tld (Postfix, from userid 1000)
                [...]

as you can see, in the upper 'Received' header both hostnames are fully 
qualified and identical, which is what i would expect.

but there is one single host, which doesn't:

        Received: from host.domain.tld (host1 [10.0.0.2])
                [...]
        Received: by host.domain.tld (Postfix, from userid 1000)
                [...]

here, the second hostname is not only not fully qualified, but is also an 
entirely different hostname.

so, how does postfix determine hostnames? and what ist the difference 
between the first hostname and the second one in parentheses?

so far i checked /etc/postfix/main.cf, /etc/hosts, and /etc/resolv.conf, 
and they are all identical (since they are provided via ansible from the 
same source).

i would appreciate any insight, and i am happy to help with any 
missing/needed information.

greetings...