It might be the "create object on layer 0" that is the problem.
Do you have multiple layers in your project?
Layer 0 would be the furthest layer usually for backgrounds and such. If layer 0 has any scrolling settings changed, then this would explain why your bullet moves at different speeds depending on how far the screen scrolls.
Either create the bullet on a different layer (instead of layer 0, you could do it on player.layernumber) or you might need to click on layer 0 on the layers window, and change the scrolling settings back to 100% x 100%
I'm taking a guess from the info we have so far, so I may be incorrect!