Look at the summary table of the Astrotracer part, we see that the tracking time when the O-GPS1 is used with a K-5 and a K-r are somehow different:
[ ASTROTRACER Tracking Time (approx. in sec.) ]
So, it simply hints that the order of magnitude for the allowable movement of the K-5 and K-r SRed sensors are different. Practically, the larger order of magnitude of the K-5's SR system should allow it to make corrections for more hand shake errors and particularly for the use of longer focal length lenses, theoretically and provided that the other parts of the SR (feedback) system can also cope with.
Update (6-8): For more detailed information of the O-GPS1, you can read this product page (Google translated, original in Japanese).
What are These Pentax Big Guns?