underscorejs render different results depending on environment when passing strings with colons to it

I'm running the framework Meteor based on NodeJS. When I use underscoreJS on my server I get some strange results. On the server this code returns true

_.contains(['s:s:s'], 's:s');

but in chromes inspector it returns false (as expected).

On the underscoreJS it says that they use the native indexOf internally. However this code:

['s:s:s'].indexOf('s:s')

return -1 both on my server and in the inspector (as is expected, but it seems strange that it renders different results if it really is used internally by underscoreJS.)

I'm not sure if this is caused by underscoreJS, NodeJS or Meteor.