Washable sofa

We bought our off-white IKEA sofa second-hand. While the colour is not ideal (it shows dirt - any dirt), it was the only comfortable sofa in the used furniture store that wasn't covered in grotesque flower patterns. Fortunately, all the cushion covers are machine washable, so whenever we get something on it (chocolate is a frequent culprit) I throw all the covers in the washing machine. After several washings though, a new problem is becoming apparent. While the cushion covers are becoming whiter and whiter, the rest of the fabric covering (which did not look particularly dirty to begin with) looks dingier and ... much less white in comparison. This is unfortunately most apparent where cushion meets rest-of-sofa.

"Do you see a difference? I see a difference."

I suppose an upholstery cleaning is in order.