In developing the P-series pistols, Ruger Engineers have taken to heart complaints about the size of the P89/P90. Subsequent models not only slimmed the gun down, but also reduced the overall number of parts; thereby simplifying the design by making fewer parts perform multiple tasks and making it less costly to produce. The continuing evolution of the P-series has led to the polymer frame guns Ruger offers today (it was tried earlier and I believe was designated inhouse as the P-92, but was never marketed) which carried the simplification further. Best of all, this was done without sacrifice in durability of the firearm.
On the P97, Ruger also uses internal rail slides which mate with the frame. This, Ruger Engineers feel, contributes to the greater accuracy of the P97 over the P90. If you look at the two guns from the muzzle end (empty of course, slide locked back), you'll see them on the P97.