I live in Gero / horrocks and work in perth 4 days a week. No wind in Perth this week, Gero looks ordinary on the forecast ... But this arvo off shore ( ie point Moore ) has > 20 knots all afternoon.
I suspect the forecast is based on the airport numbers, nowhere near as good as the port authority numbers. Gero is unreal, wish I was there now!
Cheers
The forecast was right, it was the weather that changed! 
The forecast is for coastal waters, and is usually pretty accurate; but it will have occasionally have it's off days.
Given the movement of the weather up to the point of issue, the forecast is predicting the highest probability of where the weather will head next. Unfortunately there are many variables that alter the course of the weather, so it's important to understand that a forecast is a probability, not a certainty.
This is interesting :
www.seabreeze.com.au/Articles/Miscellaneous/A-history-of-weather-discovery_1216853.aspx
Like Laurie said, the forecast that you see on Seabreeze, BOM, etc. is the one most likely (probable) to happen. But meteorological models can't represent exacty, at all moments, the chaotic atmospheric dynamic system.
Models try to be accurate, but errors are introduced by the use of imperfect initial conditions (that are impossible to know exactly...), amplified by the chaotic nature of the dynamical system and errors introduced because of imperfections in the model formulation and equations. There's also uncertainty on the observed data used in the models...
Be more forgiving 
Yes, I appreciate the forecast is just a forecast. Its pretty good.
My observation is though, that the seabreeze forecast seems to correlate well with actual observations at the BOM weather station at Gero airport which is 5k inland. The wind at Point Moore which sticks out 2 or 3k into the ocean is often 5 knots or so stronger. Which is very good! 
Try getting a kite in Sydney at the moment .No wind for weeks now and the forecast still looks like sh!t for the next week. Aarrhh i need wind !!!!
I've found looking at wind charts do my head in, mother nature delivers when she wants, just be sure to have your kites at all times,
