I am having the same issue but only on 3 machines out of 1500+ Dell machines. I have not gone out and seen if the machine actually shows that in the BIOS. I also have a Machine that I beleive had it's motherboard replaced that reports back it's service tag as "FIELD".
I know there is a tool out there to update it, but the 3 machies with your issue and the FIELD ones are in a remote location, so not wanting to make a trip out just to check/fix.