CallContext vs ThreadStatic

Items stored as ThreadStatic are available to more than one request. IIS reuses the thread after a request is complete to process subsequent requests, it can even swap a request from one thread to another during processing. ASP.Net clears down the CallContext after each request.

Very often a request will use the same thread throughout, but it certainly won't always be the case - ASP.NET exhibits thread agility. There's an old in-depth blog article about the matter from 2005, but as of .NET 4.5 things are rather better.