Any gun made in smaller quantaties will maintain it's value more than one made by the millions. Ruger 22/77's are a very small part of Ruger's manufacturing. It is no surprise that they would hold their value more than a centerfire version when many more of the centerfires were made. The more rare the chambering, the more they hold their value. A used 6mm should sell for much more than a 30-06. But the 10-22 is always going to be less expensive than either.
You get the same thing when you look at Marlin. The 39's typically sell for a lot more than a common 336. But model 60's are dirt cheap on the used market.