It definitely isn't free - all shop bought soups have a Syn value. They don't qualify for any free food allowance, so it's a straight 1 Syn for every 20 calories.
There are 27 calories per 100g and the whole pot is 500g. So 27 x 5 = 135 calories. 135 /20 = 6.75, which would be rounded up to 7 Syns.
I suspect that will be because you put carrots or something as the free food? But things like this don't qualify for any free food, it's the same with sandwiches etc.