Understanding TTL in "rndc dumpdb"-output

[Tom]

Hi

After querying my resolver for "testbla11.example.com", I receive a 
NXDOMAIN response with a minimum-ttl (in the soa) of 3600.
When I afterwards dump the cache of my resolver (9.12.2-P1) with "rndc 
dumpdb" and look for the negative ttl, then a value much bigger than 
3600 is shown (608363):
# grep testbla /var/named/data/named_dump.db
testbla11.example.com.	608363	\-ANY	;-$NXDOMAIN

This number decrements every second.

What is this number? The same behavior for positive answers too. The 
A-record for "www.google.com" has a TTL for 300 seconds. In the "rndc 
dumpdb"-output I have a value for 605082.

Any hints?
Thank you.

Kind regards,
Tom