What leak can cause an instance of TimeBoundedCache.ExpirableItem to leak in WCF?

I profiled my WCF application with the .NET Memory Profiler and found that there was a leak of instances of the types TimeBoundedCache.ExpirableItem and Byte []:

The comparison was performed using two images with an interval of 1 hour, and comparisons at different times also show an ever-increasing number of two types. Apart from these two types, there is no obvious leak.

The distribution stack is as follows:

Does anyone know what could be causing this?