Varnish Inconsistently Evicting Pages from Cache

In our varnish config we have a vcl_fetch function defined as follows:

if (beresp.ttl > 0s) {
   /* Remove Expires from backend, it's not long enough */
   unset beresp.http.expires;

   /* Set the clients TTL on this object */
   set beresp.http.cache-control = "max-age=900";

   /* Set how long Varnish will keep it */
   set beresp.ttl = 1h;

   /* marker for vcl_deliver to reset Age: */
   set beresp.http.magicmarker = "1";
 }

Then in vcl_deliver:

if (resp.http.magicmarker) {
   /* Remove the magic marker */
   unset resp.http.magicmarker;

   /* By definition we have a fresh object */
   set resp.http.age = "0";
}

The issue is that if I setup a script (either a shell script or a php script) to hit the same URL, I first see a MISS, followed by a HIT (all good so far).

What I am expecting to sit is repeated HITS until the 1 hour mark has passed, one MISS at the 1 hour mark, followed by an hour of HITs again. In other words because I'm hitting the same URL repeatedly it should cycle between HIT and MISS every 1 HOUR.

Instead what I see is something like this:

 6/20/2013 6:30:05 - STRING HIT NOT FOUND
 6/20/2013 10:32:11 - STRING HIT NOT FOUND
 6/21/2013 12:33:06 - STRING HIT NOT FOUND
 6/21/2013 2:34:06 - STRING HIT NOT FOUND

It's now 5:57AM (almost 4 hours since the HIT not found).

The cache misses above do all appear around the half hour mark but they're 4 hours, 2 hours and 2 hours apart and the TTL is set to 1 hour.

So I don't understand why the object is being kept longer than 1 hour and why its evicted at seemingly random intervals. How can I better determine whats going on?

By the way I am using varnish 3 if that helps.