Why are some Protractor tests skipped?

When I launch protractor conf.js and this file needs to launch multiple spec files (like below), some of them are (randomly?) skipped. It works quite well when I run each spec separately.

specs: [
  'tests/test-1-*.js',
  'tests/test-2-*.js',
  'tests/test-3-*.js'
],

I've found a workaround, but now I would like to know the root cause... Any idea?