Some comps house rule that extra attacks do not generate more extra attacks. So, say a Ripperdactyls attack hits and another attack is rolled. This attack hits, and RAW another attack is rolled. If that attack hits yet another attack is rolled, and so on. Obviously when an attack hits on 4+ with rerolls this can mean a single initial attack turns into quite a lot of wound rolls. That is how PPC works, I suspect that SCGT house rules that after the first extra attack no new ones are generated; this means a single attack can I not every create 2 wound rolls maximum.
I also believe SCGT is strictly formula based as opposed to PPC which modifies formula values with play testing input. Also they may be differently scaled. For example, if a Chaos Warrior is 16 points in one comp and 160 in another they could still be the same effective value, the latter comp may just be using points values 10x as big for everything.
[Edit] I checked and SCGT poses a penalty on extra attacks, so Ripperdactyls are correctly costed in being cheaper with their comp since they literally aren't as good.
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