50g of fruit in 100g of jam definitely looks like 50% fruit as claimed.
But if you start with 60g of sugar and 50g of fruit, that represent 45% of the original weight – and that ignores any water added to facilitate the boiling of the fruit.
When I have made jam and apple jelly, I have never really considered the chemical make-up or percentages but I have observed steam which I always assumed was the added water and some water within the fruit.
If some liquid from the oranges has evaporated in the jam-making process and if the sugar does not evaporate, then the percentage of fruit has reduced below the starting 45%.
Next year, I will try to enhance my vocabulary to include mathematical and chemical terms. This year I had toast and marmalade – and nice it was too.