With some brands' cartridges, you can flip them 180-degrees to fix backwards supply lines. Do not know if yours is one of them. Apparantly, the one you took out was. It is also possible that your valve is installed upside down. Some are designed for a tub and when using it for a shower, you still use the bottom outlet, or move the plug to the other side.
I've read that this tends to happen with back-to-back installations...one of them will be backwards unless you do so fancy cross-overs with the supply piping.
You may need to swap things around.