buying and selling

Just wondering if someone can clarify something for me.
I guess it may not be SDL specific, but I see this all the time in relation to the SDL shares.

I only have the westpac traders account so I'm not sure if this is as detailed as other trading tools.
What I don't understand is this........

My buy and sell chart shows 1,600,000 bids to buy shares at 56.5 cents. The sell side is undersubscribed by nearly 1,000,000 at that price. Why then would someone post a sell of 250,000 shares at 55 cents? Why would anyone short themselves 1.5 cents by 250,000 shares.

Confused, can anyone explain, please.
Thanks in advance.