In addition...
If you need to call Microsoft for support they will ask for the license
number and then refer you to the company that sold you the OEM version. The
higher price of the full retail version presumably includes an element
Microsoft believe covers their costs for providing support. If OEM vendors
believe they don't need to charge for support they are free to sell the OEM
version of the OS cheaper than MS sell the retail version.
It's a bit like choosing where to buy a car. Company x charges a higher
price because they give free servicing for the first three years.. Company y
charges a lower price because the don't give free servicing. It's the same
car but a different deal and it's (usually) your choice which you go for.
If you were told you were getting the full retail version and paid the
"retail price" but were actually given the OEM version you should complain
to the company.
Some vendors do a similar trick with hard drives.