It's been a while since I've used one but from memory you can get ferning at different stages during your cycle because of hormone surges. The thing to note is that the ferning has to be at it's thickest and also not have the pebble pattern for you to be ovulating.
Also strange things can happen and your body can throw in a short cycle. The cycle Caleb was conceived I O'd on CD 12 which was the first time I'd ever O'd that early!