regarding www.yahoo.com (akamai) TTL values

[Vishwas]

Hi All,

My understanding is that TTL values for all entries specified in a
zone are equal to the TTL value specified in the SOA section of that
zone.

Following is the scenario that is confusing me.

STEP-1 [get current TTL from local DNS, and find authority for www.yahoo.com]
$ dig www.yahoo.com
...
;; ANSWER SECTION:
www.yahoo.com.          139     IN      CNAME   www.yahoo-ht3.akadns.net.
www.yahoo-ht3.akadns.net. 60    IN      A       209.131.36.158

;; AUTHORITY SECTION:
akadns.net.             17543   IN      NS      za.akadns.org.
...

STEP-2 [ask the authority about an entry in its zone and observe the
TTL value returned]
$ dig @za.akadns.org www.yahoo-ht3.akadns.net
...
;; ANSWER SECTION:
www.yahoo-ht3.akadns.net. 60    IN      A       209.131.36.158
...

STEP-3 [check the SOA setting of the authority]
$ nslookup
> set type=soa
> akadns.net
Server:         192.122.134.35
Address:        192.122.134.35#53

Non-authoritative answer:
akadns.net
        origin = internal.akamai.com
        mail addr = hostmaster.akamai.com
        serial = 1177489512
        refresh = 19200
        retry = 19200
        expire = 19200
        minimum = 180

Authoritative answers can be found from:
akadns.net      nameserver = za.akadns.org.
...