Bidding notrumps often shows a balanced hand, but not always. A response of 1NT to a suit opening doesn’t need to be balanced, it just shows about 6-9 points and tells partner you don’t have a suit that you can bid at the 1-level. In fact, you can respond 1NT on some quite unbalanced hands! Better to do that than bid a new suit at the 2 level and get too high.
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