Ah Phoi--a few more sen worth....
Some people are just more buoyant than others. Some of it is bone density, some of it is biotype and how "curvy" the diver is (curvier divers usually need more weight than ones with less on the "bottom" "top" and "middle" sections--or the 'three B's': butt, boob, belly). And of course, as HoleMaster points out, some of it is gear--in addition to what HM lists, there is also the question of wetsuit thickness and size--the more neoprene you wear, the more you float. It's a very individual thing.
Others have answered the question of what to do if you loose your weight belt, so I'll just summarize:
1) keep your airway open (don't hold your breath, but you can breathe)
2) flare your body out horizontally, like skydivers do before they open their chutes. This will slow you down a little bit.
If you are very worried about the speed of your ascent, monitor your health for a couple of hours and immediately begin breathing 100% oxygen to help eliminate the residual nitrogen a bit faster, and retard big bubble formation.
Avoiding losing your weight belt:
1) Some weight belts have sort of loose buckles. Try to find one that has a nice, clampy buckle, or buy your own and take it with you along with your mask and other equipment when you go dive. That way you know it's in good condition. I personally think the ones with metal buckles are better than the ones with plastic buckles.
2) Keep in mind that when you get to depth, your tummy will be compressed a little bit from the pressure, so your belt may feel looser. You can check it when you get to the bottom and tighten it so it doesn't slip around and maybe put the buckle in a position that it can be caught on some part of The BCD.
3) You might consider buying a weight harness if you are very concerned about the belt slipping off. Some guys, especially, who have round "middles" find that the weight belt slips down past their hips. The harness is something like a pair of braces in that you wear it over your shoulders. It has quick release catches so that you can drop the weight easily in an emergency.
4) When you buy your next BCD, consider getting one with integrated weight pouches. These also have quick release catches so that you can dump them if necessary.
