Well, with Dialga and Giratina, I advanced to frame 498, and right after that I walked into that house where that man runs out. From that point, my HG advances 6 frames until I reach the point I can choose between the legends. There my frame was 504, got the Pokémon on 505 with contains my wanted spread.
With Palkia, I tried 4 times (lol). The first time, advanced to 498, I landed on 505, second time, I thought hm, maybe I should advance to 497, so I did that, and then I landed on 503 >_< And the final time, I advanced to 497 again, and then I got the right Palkia.
I really don't understand that