prefetch and serve-expired statistics

[spam at wilysloth.com]

I am trying to understand my prefetch and serve-expired statistics. The
relevant parts of my unbound.conf is as follows:

prefetch: yes
serve-expired: yes
serve-expired-ttl: 86400

total.num.queries=95364
total.num.queries_ip_ratelimited=0
total.num.cachehits=78438
total.num.cachemiss=16926
total.num.prefetch=54883
total.num.zero_ttl=61509
total.num.recursivereplies=16926

You will notice that prefetch + zero_ttl exceeds the total number of queries so
they must overlap. My guess is that every zero_ttl hit triggers a prefetch. So
every zero_ttl increment is a prefetch increment but not every prefetch
increment is a zero_ttl increment (i.e. < 10% TTL but not zero). However, if
this were true prefetch would always be bigger than zero_ttl.

As this is not the case, some questions:
 - What type of query increments prefetch and zero_ttl?
 - Is that prefetch number correct? 57% prefetch looks awful high.
 - Could the prefetch and zero_ttl statistics be reversed? If all zero_ttl
   triggered a prefetch then 61509 - 54883 = 6626 (7%) for non-zero_ttl looks
   believable.

Thanks.