Florescent fixtures wattage are based on tube length. I'm betting they ordered the wrong length....but either way, yes you can use it. It will be brighter than normal and the bulb won't last as long.
In the planted tank aquarium hobby this is a little trick we used to provide more light for plants off of current fixtures. You take a ballast for a 48 inch fixture and replace the ballast in the 18 incher. It's called "over driving" I've done it and still have a few fixtures down like this. They work fine, but like I said the bulbs don't last long. In my case I have to replace the bulb every six months, but not because it's light is degrading (this is why the bulbs in planted tanks should be replaced every six months any how) but because the freaking thing is toast. lol