I don't know why or how, but I always got a refund from the US (and my state) for income earned in the UK. At the time, I didn't have a particularly complicated tax situation, no kids, no mortgage for most years, no investments outside of 401Ks. I never questioned it with the accountant and just figured that was what I paid him to do.
I don't want to cast doubt on your return, as I am sure it depends on your income and various other factors, but may be worth just double checking your inputs one last time.