What are the ingredients? Perhaps the person quoting the 8 syns for the whole cake is counting the Weetabix as a HEX whereas the 3.5 syns version isn't? Can't tell unless we know exactly what the ingredients/quantities are.
2 Weetabix = 6.5 syns or 1 HEXB
113g raisins = 16 syns
113g flour = 20.5 syns
sweetener = Free
140ml milk = 2 syns if skimmed (obviously need to edit this if use semi-skimmed, whole, soya or whatever)
So syns-wise that particular cake is 45 syns in total or 36.5 if you use the Weetabix/milk as part of your HEXes.
So divided by 12 slices makes about 3.5 or 4 syns per slice (3.75 to be exact).
Stuff like this you're better off just counting as syns tbh, as to count as a whole HEXB (for the Weetabix) you'd have to eat the entire cake (and thus the other 38.5 syns!! ) or count each slice as 1/12 (0.083...) of a HEXB and 1/12-of-a-third (0.02777...) of a HEXA etc etc which gets a bit ridiculous...