ThreadPool max threads
Firstly, your "knowledge" of the defaults is incorrect. The limit of 25 threads per processor was back from .NET 1.1. It was increased in .NET 2, and now:
Beginning with the .NET Framework version 4, the default size of the thread pool for a process depends on several factors, such as the size of the virtual address space. A process can call the GetMaxThreads method to determine the number of threads.
However, there's something else at play: the thread pool doesn't immediately create new threads in all situations. In order to cope with bursts of small tasks, it limits how quickly it creates new threads. IIRC, it will create one thread every 0.5 seconds if there are outstanding tasks, up to the maximum number of threads. I can't immediately see that figure documented though, so it may well change. I strongly suspect that's what you're seeing though. Try queuing a lot of items and then monitor the number of threads over time.
When demand is low, the actual number of thread pool threads can fall below the minimum values.
Read this too: Patterns for Parallel Programming: Understanding and Applying Parallel Patterns with the .NET Framework 4
Firstly check this link, especially this remark:
If the common language runtime is hosted, for example by Internet Information Services (IIS) or SQL Server, the host can limit or prevent changes to the thread pool size.
Then you should check the return value of ThreadPool.SetMaxThreads(threads, threads) method. Maybe it returns false?