In a code, "CAB" is written as "6". Om thinks this is "number of letters × 2" (3 letters × 2 = 6)

Why

Om found a rule that genuinely produces the right number for THIS one example — 3 letters times 2 does equal 6 — but a rule matching one example by coincidence is not the same as the rule actually being used. His sister's rule also gives 6 for the same word, through a completely different calculation: C is the 3rd letter, A is the 1st, B is the 2nd, and 3+1+2=6. Both rules land on 6 here purely by coincidence, since this particular word happens to make both calculations agree. Test it on a different word to break the tie: for 'CAT', letter-count×2 gives 3×2=6, while adding alphabet positions gives C(3)+A(1)+T(20)=24 — two very different answers, revealing which rule was actually meant only once you check a case where they disagree. His sister is right: the real rule adds up each letter's position in the alphabet — when two different rules happen to agree on one example, that example alone cannot tell you which one is real; only a case where they give different answers can.