Well, I tested this on my PSR-E433, and the Portable Grand button does NOT change the split point. But, yes, selecting a registration WILL change the split point if the registration has a different split point stored in it than the split point that you set manually. That's probably what is happening. Even if you are not using the registrations when playing, if you manually set a split point, and then hit a registration button afterward, even if you then hit the Portable Grand button again, the split point will now be set to whatever was stored in that registration.