From page 100 of the FileMaker Pro 13 Help (http://fmhelp.filemaker.com/docs/13/en/fmp13_help.pdf):
"In networked databases, global field changes made by a guest are available only to that guest, and only until the guest closes the file. Each guest can have a different global field value. If all guests must have the same value, define a calculation field with a constant value (in quotes)."
And on page 184:
"If your file is shared, only the host's changes to global field data are saved. Changes are saved only when the file is closed."
Or, as you have discovered, create another file with one record that contains the variables with the default values.