I've found the problem. It seems that shouldn't define properties in your plugin root like so:
;(function ( $, window, document, undefined ) {
$.widget( "namespace.mywidgetname" , {
myPrivateVar = {},
_create: function(){
this.myPrivateVar.test = 2;
....
This keeps a reference for all the instances. But instead you should declare your variables right inside the _create method.
;(function ( $, window, document, undefined ) {
$.widget( "namespace.mywidgetname" , {
_create: function(){
this.myPrivateVar = {};
this.myPrivateVar.test = 2;
....