Sainsbury's basics baked beans

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This might sound a bit odd but for the last couple of years I've always made sure that the only baked beans I buy are sainsbury's basics (I've only ever bought supermarket own brand beans). The reason is that the calorie content is so much lower than Tesco, Asda or Morrisons value ranges, even though the ingredients (and percentages) are more or less identical.
Sainsbury's basics are 49 cals per 100g whereas Asda smartprice are almost double that at 96cals per 100g! Tesco value are 90cals per 100g.
It seems a bit too good to be true that basics would taste exactly the same but somehow be half the calories than the rest, do they have a good reputation for keeping their nutritional info correct and up to date or is it possibly an error in labelling?