You need to look at the calories - they are 1 syn for every 20 calories. The Tesco one is not a real cranberry fruit juice but is so low in calories that the syns become neglible to the point it is syn free.
Thanks. Just looked and the one I have is 8 calories per 100ml and Tesco is 4 calories per 100ml but Tesco has 10% cranberry juice and mine (Ocean Spray Light) has 25% cranberry juice.
I'll carry on synning it I think as it tastes nicer and has more actual juice