Using Meteoric to deploy Meteor.js, MongoError: auth fails

Question

When using Meteoric to start the meteor app that connects to a mongodb server that has auth enabled, first the variable is set:

export MONGO_URL = mongodb://admin:password@localhost:27017/myAppDb

Meteoric is configured as shown:

PRE_METEOR_START="export ENVIRONMENT=prod MONGO_URL=mongodb://admin:password@localhost:27017/myAppDb;"

Then meteor is launched using the command meteoric deploy The error appears:

MongoError: auth fails

Question: Why is meteor unable to access mongodb using meteoric deploy, but when launching meteor using mrt, mongodb access works fine?

I do notice that the Meteoric script contains the following code, what does the -z flag do?

if [ -z "$MONGO_URL" ]; then
    MONGO_URL=mongodb://localhost:27017/$APP_NAME