The website regarding Weetabix gives the carb value as 68.4g per 100g. They also quote a serving as being 37.5g so that figure I quoted is correct for that quantity.
If one Weetabix weighs 20g then it's carb content is as you say close to 14g carbs (13.68g).
Carbohydrates convert 100% to glucose when ingested and it is the TOTAL carb count that matters, that figure includes any sugars which are ADDED sugars. If you only count 'sugars' you will seriously underestimate the amount of Insulin required for any meals.
Hope that is clearer.