I would suspect that this has to do with the different options for the form's enctype when the method is POST. This controls the way the parameters are delivered to CGI, from memory:

• not-defined => "param1=one+value¶m2=another+val"
• "text/plain" => "param1=one value param2=another val"
• "multipart/form-data" => "param1=one value\nparam2=another val\n"

My guess would be that since the default is to replace all spaces with "+" the CGI parameter processing by default does the reverse