A device is usually calibrated over its entire measurement range. Is your anemometer always - 9 units across its entire measurable range? What if your measured value is equal to 9mph or less?
If your device is that much different, then should you really trust it? In the engineering world, the item is adjusted until it is within an acceptable tolerance of error across the entire range. You would not usually slap a sticker on said device saying "subtract 9 from measured value" as that's considered bad practice.