I had the same issue with my K-7 way back when, Bob. It's a factory calibration thing and not accessible to users. In the end, I simply put up with it and mentally factored in the difference.
It's not uncommon with other brands too. I have a Panasonic DMC-TZ70 compact where both the left-to-right and front-to-back tilt were out. Thankfully, I managed to buy service documentation that provided the (rather convoluted) approach for accessing service menus and performing the re-calibration with a home-printed target... but, really, Panasonic doesn't want you to do it either. Search around on Google for "horizon level inaccurate <brand name>" and you'll find examples of the problem for most of the brands.
My recommendation is, if you know the level indicator is off by a couple of degrees, just factor that in when using the level. Sending it in for service won't be cheap, and risks more harm than good for such a minor concern (even if it is a little bit annoying at first
)...