I guess if you just look at the first ingredient but its both obvious there are multiple sources of sugar/simple carbs and that jam is typically very heavily sweetened; this is how it has a long shelf life.
For example, the very first recipe for strawberry jelly from some horrible spam recipe site:
2 cups (16 ounces) pure strawberry juice (I used 3 1/2 pounds ripe, fresh strawberries to get this amount of juice) 3 1/3 cups (666 grams) granulated sugar
It's literally more sugar than strawberry juice.
1. adding more sugar
2. cooking it for a longer time
3. adding pectin (either store-bought or from natural sources)
Of course it depends on the kind of fruit, but it’s quite rare to have more sugar than fruit, even without pectin or longer cooking time.