"Cache Total Entries" perf counter doesn't show current total

I am using trying to use perf counters to monitor the HttpRuntime.Cache to see how many items are currently in the cache. This is based on:
http://msdn.microsoft.com/en-us/library/fxk122b4(v=vs.100).aspx
However in Performance Monitor I see the "Cache Total Entries" briefly increase for about a second as I insert into the cache but then it goes back to zero again, i.e. it seems to show a delta not a total.
As a test I'm doing this in a button onclick handler of a brand new asp.net site:
HttpRuntime.Cache.Insert(Guid.NewGuid().ToString(), "Value 1234", null, DateTime.MaxValue, TimeSpan.FromMinutes(5), CacheItemPriority.Default, null);
If I break into the code and look at HttpRuntime.Cache.Count it appears to reflect an accurate number.
Am running Win2K8 R2 SP1, ASP.NET 4.0. Any thoughts appreciated.
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