i think Tower has two warehouses in the US, on opposite coasts or one might be in central USA. So when the warehouse that tries to process your order does'nt have a part they have to get it transfered to them from the other warehouse before posting it to you here in Aus. And the domestic mail service in the US has alot of issues (security, etc) which slow it down.

I could be wrong but thats what i remember my old man saying when he was dealing with Tower alot. I might ask him and get back to you all.