Report Server URL Error - Parameters

One of the most common mistakes when linking to a SSRS url and adding parameters is...

Parameters are case-sensitive.  Check to make sure the case like "OrderID" is the same in the url as is in the report.  If the case does not match then the report will not work.

Why is this?  XML (used in the guts of the rdl definition) is case sensitive so you have to match case.