Avocados are a good example of why it's difficult to accurately count carbs when measurements are given in increments like one-half. The carb count will depend upon the weight and variety of the avocado. The Nutrient Database from the USDA lists three types of avocados — all with slightly different level of carbs. I believe that Fitday has adapted their information from this database. I have found it common for package labels to list amounts that differ by small, or large, amounts. Part of the problem is the way producers are allowed to round values down so that small portions may be zero or less than one gram when a reasonable portion would contain many grams of carbs.

If you'd like to try using the USDA database, I've put a simplified version of it on one of my pages. Look for Instant Carb Counter near the top of the page. Or you can goto the UDSA site and access it from the net, or download a version that works in a Palm Pilot — very portable and convenient. (I'm currently over in France cooking in a Michelin-starred restaurant and the portable version has been very helpful in the kitchen.)