Probably badly phrased, but I guess she wanted to say that if you ban high-capacity ammunition magazines now, then in the future there will be less around: If no new high-capacity ammunition magazines can be sold and bought anymore, the stock of existing ones will decrease over time because of old ones that are given back to the authorities, malfunctioning ones, etc. - therefore probably reducing events where such super-deadly weapons can be used.
Her spokesperson clarified what she wanted to say.
"The congresswoman has been working on a high-capacity assault magazine ban for years and has been deeply involved in the issue; she simply misspoke in referring to 'magazines' when she should have referred to 'clips,' which cannot be reused because they don't have a feeding mechanism," Johnson said.
Actually, clips in most guns can be reused as well.
