I think the hitbox of the edge is slightly to the left of the model, so if you're just guessing the throw , it seems to be slightly to the left of the edge.
When guessing 3 things can happen,
You didn't swing out wide enough and it will bounce off the first edge and then hit the pillar, causing it to bounce back again and block you off from leaving palace.
You aimed to far to the right, and the molly bounces to the right and you get blocked from leaving palace.
You aimed to far to the left, then the molly bounces onto site, which won't block you from entering site.