In most cases, if you go to Settings>Regional and Currency Settings,
you should notice that Enable Auto Currency Conversion By Visitor
Origin: is selected. This means that if the customer is in a country,
for example the US, the figure will may be in $US if that is one of the
available currencies selected.
If the Available Currencies on Currency Converter: is not in the location of the customer, $US is the default value.
Some customers may also use proxy networks (i.e. in simple terms, there
work is routed through to another country, so it looks like the browser
is in another country), which may effect the currency shown.
If you always want it to show as the Store Default Currency:, then deselect Enable Auto Currency Conversion By Visitor Origin:
