Invalid reverse?

[lamerfreak]

H'lo,

In troubleshooting a mail issue, I came across some strange DNS, and
I'd just like verification whether it's valid or not (my tendency,
based on some prliminary research, says it's not, but...)

One IP in question is 12.152.168.13 -

~$ dig -x 12.152.168.13                                       

; <<>> DiG 9.3.1 <<>> -x 12.152.168.13
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4836
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;13.168.152.12.in-addr.arpa.    IN      PTR

;; ANSWER SECTION:
13.168.152.12.in-addr.arpa. 172603 IN   CNAME
13.0/25.168.152.12.in-addr.arpa.
13.0/25.168.152.12.in-addr.arpa. 3403 IN PTR mail.achieveglobal.com.

The CNAME I believe isn't valid in this context, but the PTR is.  I'm
not sure what the "13.0/" is supposed to signify, either...

Authoritative servers for it are listed as:

0/25.168.152.12.in-addr.arpa. 3403 IN   NS tyke.achieveglobal.com.
0/25.168.152.12.in-addr.arpa. 3403 IN   NS alta.achieveglobal.com.

alta refers me to the root servers, tyke refers me to some ATT.NET
servers, which respond as noted:

;; AUTHORITY SECTION:
12.in-addr.arpa.        63795   IN      NS CBRU.BR.NS.ELS-GMS.ATT.NET.
12.in-addr.arpa.        63795   IN      NS CMTU.MT.NS.ELS-GMS.ATT.NET.
12.in-addr.arpa.        63795   IN      NS DBRU.BR.NS.ELS-GMS.ATT.NET.
12.in-addr.arpa.        63795   IN      NS DMTU.MT.NS.ELS-GMS.ATT.NET.

~$ dig -x 12.152.168.13 @CBRU.BR.NS.ELS-GMS.ATT.NET +norecurse

; <<>> DiG 9.3.1 <<>> -x 12.152.168.13 @CBRU.BR.NS.ELS-GMS.ATT.NET
+norecurse
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39912
;; flags: qr aa; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;13.168.152.12.in-addr.arpa.    IN      PTR

;; ANSWER SECTION:
13.168.152.12.in-addr.arpa. 172800 IN   CNAME
13.0/25.168.152.12.in-addr.arpa.
13.0/25.168.152.12.in-addr.arpa. 3600 IN PTR mail.achieveglobal.com.

;; AUTHORITY SECTION:
0/25.168.152.12.in-addr.arpa. 3600 IN   NS tyke.achieveglobal.com.
0/25.168.152.12.in-addr.arpa. 3600 IN   NS alta.achieveglobal.com.

Am I missing anything?  Anyone have an answer as to what they're
trying to do with the reverse assignment?