When explaining bare shaft tuning, either most don't understand, or forget to mention that arrows that show weak can also be too stiff. Now generally, an arrow that shows stiff is almost always stiff. But an arrow that wants to show stiff but can't clear the bow will bounce off the bow and kick to the weak side giving a false reading.
Due to the extremely high percentage of guys using arrows that are too stiff, I would venture an educated guess that the false reading is much more common than an accurate reading. I can't tell you how it is possible that shafts that are too stiff can bounce of the bow and give you a good bare shaft read, but this is fairly common as well.
Unaware of the Spyder bows shelf cut.. center or past.. but if you are not drawing to 29"s, you will likely need 600's except in the axis.. that would be a 500.
BigJim