Net session command shows IP address

When I use the "net session" command some
of the logged in users computers are shown by
name and some by IP address.

Does anyone know why this is happening? Could it indicate a problem
with name resolution on the network?

Computer User
-------- ----
\\x.y.1.177 EL
\\x.y.1.2 MC
\\x.y.1.207 LO
\\x.y.1.207 LO
\\x.y.1.33 RI
\\CA
\\CP RI
\\QWE-D330-01 DO
\\QWE-TIN SC
\\QWE-WS27 JA
\\QWE0017 AL

Thanks.