与直接运行相比，npm审核在安装后的结果有所不同

Question

从标题来看，安装后自动运行的npm与直接运行时的npm有所不同：

added 1 package from 1 contributor in 3.609s
[!] 9 vulnerabilities found [5693 packages audited]
    Severity: 2 Low | 5 Moderate | 2 High
    Run `npm audit` for more detail

$ npm audit

                       === npm audit security report ===

[+] no known vulnerabilities found
    Packages audited: 164 (0 dev, 0 optional)

我认为这可能是因为它扫描了全局软件包（5693个软件包与164个软件包），但是我无法进行全局审核：

$ npm 

audit -g
npm ERR! code EAUDITGLOBAL
npm ERR! `npm audit` does not support testing globals

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/benp/.npm/_logs/2018-07-12T10_32_42_261Z-debug.log

这是怎么回事？