With the circumstances you have explained, your DUI conviction from 10 years ago is not the felony on your record. Recently (July 2007) the legislature promulgated a statute making a fifth DUI conviction within 10 years a felony. The prior DUI conviction you had was a gross misdemeanor under the statute and the maximum penalty on that charge is 365 days and a $5000 fine. DUI convictions do vary by circumstance state-to-state in whether they're considered a felony or a misdemeanor.
I also wonder if there was any Juvenile criminal history that was a felony? It would be a good idea to talk with a criminal defense attorney in your area so you can see what is actually consists of your criminal history.
I agree with the previous posts, and would add that there is always the possibility that another's record has been superimposed into yours or that you share the same name and/or date of birth as another individual who has a record and that is how this mistake came about.
In 2006, the Washington legislature changed the DUI laws to make certain DUI's a felony.
A charge of DUI or Physical Control can be a felony if the defendant has had four or more "prior offenses" in the past ten years. "Prior offenses" are defined as convictions for DUI, Physical Control, Negligent Driving 1st Degree, Reckless Driving, or Reckless Endangerment, where the charge was initially filed as DUI or Physical Control. Successfully completed deferred prosecutions also count.
Additionally, a DUI or Physical Control charge may become a felony if the defendant has a prior (lifetime) conviction for an alcohol related Vehicular Assault or Vehicular Homicide.
If your circumstances don't fit the bill, then most likely your DUI was not counted as a felony, but rather as a DUI misdemeanor.
Typically, your run of the mill DUI is a misdemeanor and not a felony in most states. In many states, if you keep racking up DUIs, like a third DUI, then the DUI can be charged as a felony. If there is serious bodily injury or death, the DUI will be charged as a felony. You may inquire as to whether your DUI is eligible for sealing or expungement (removal). You should check with a DUI lawyer in your state to see what's on your criminal record, and whether it can be removed (expunged).