The breed standard is simply a that -- a standard -- used for show dogs really, and while it should be where a cavalier fits in -- kind of (see below!) -- it is truly meaningless in terms of determining a healthy weight for a given dog. This is extremely important to understand or a dog's health could be at serious risk!!
A dog is the right weight at whatever the healthy weight is for that individual dog -- and only in a very very rare situation would it ever be the case that a 21lb dog should actually weigh 12 lb! It would have to be grossly, grossly obese (several years of rescuing cavaliers, and I have never seen a dog that needed to lose half its weight!! About a third is the most, and that has been very rare. Though one of mine was rescued at 18lbs and needed to drop to 12.5!).
The bottom of the breed standard in the US is 13lbs, not 12lbs, so 12 lbs would be underweight for US show dogs anyway. However, very very few cavaliers -- show or pet -- are way down at that bottom end of the scale anyway. I have seen almost none in several years of doing rescue.
If a dog is the right weight for its frame at 21lbs, even at 44 lbs (and I have had rescue cavaliers come in that large! And NOT overweight for their frame at all
) then that is its 'correct' weight. Just like people -- there's a huge variation in healthy weights for height, depending on a person's build and whether they have a lot of muscle (which weighs more than fat).
It is actually more common for cavaliers, including show cavaliers, to be up at or just over the breed standard these days -- I just spent a week with breeder friends who pointed out that males are only rarely within breed standard any more; most are up around 20lbs-plus. There is a growing feeling too amongst some researchers and breeders that smaller cavaliers are all else being equal, running more health risks and probably passing along more genetic risk known to be associated with breeding for miniaturisiation (almost all the breeds afflicted with
syringomyelia for example are miniaturised -- toy breeds -- bred for smaller skulls and flatter faces/smaller muzzles than would be normal) and that a larger cavalier would likely be a healthier dog. Thus it is very important to avoid anyone deliberately breeding extra small cavaliers and I expect the norm at some point in the future will be a breed pushing more into the 20s rather than the 12-18lb range that stands now.
Th best way to judge a dog's direct weight is 1) consult with your vet, and never put a dog on a diet simply to meet a brand standard; and 2) look and aim for a healthy waist, but not a thin waist. See
http://roycroftcavaliers.com/manualfeeding.htm which has great information and some images of correct weights and waists.
Hope that makes sense!