Usually that means the previous owner did not get the proper permits when making those changes. Not always...but usually. Check your house detail on the County site by accessing the King County Parcel Viewer.
Many sites pick their info off the County Records vs the Listing detail. If you ask the County to "change" the # of bedrooms and baths, they may require permit fees AND to get them permitted today in hindsight, they likely will require the improvements be up to current code requirements. Could be a big expense involved if the venting and window size in the bedroom(s) are not to current code requirements. That often happens on a lower level vs upper level bedroom and bath.
If the "current info online" does NOT match the County Records, then the answer to your question is much easier. But more than likely, the County Records will match that online info.
If I understand what you are saying when you bought the house it may have been listed as a 3 bedroom 2 bath for example but a 4th bedroom and additional bath have been added, but the county records don't reflect them. Is this correct?
By correcting the county records you will have the benefit of having a more accurate property value reflected on Zillow & other automated valuation sites, but you may also have your property values increased in the eyes of the county for tax purposes.
If you intend to sell, a good agent will be able to provide accurate comparables to what your home truly is regardless of the tax records. You can also go into Zillow & other sites and claim your home and amend their data to better reflect the current size & room count.
Finally, you can contact the assessorâ€™s office and discuss the discrepancies with them. I've included a link below. I hope this was helpful.