well, that's how a listpicker behaves, what best you can do is instead of making ThemeListPicker_SelectionChanged make a parent stackpanel inside the datatemplate somewhat like this
<Listpicker.ItemTemplate>
<DataTemplate x:Name="PickerItemTemplate">
<StackPanel tap="stk_Tap">
<TextBlock/>
</StackPanel>
</DataTemplate>
</Listpicker.ItemTemplate>
<Listpicker.FullModeItemTemplate>
<DataTemplate x:Name="PickerFullModeItemTemplate">
<StackPanel tap="stk_Tap">
<TextBlock/>
</StackPanel>
</DataTemplate>
<Listpicker.FullModeItemTemplate>
now use this tap stk_Tap to do your action as, this event would also get called every time the selection changed gets called but, it wont exhibit the buggy behavior like that of selection changed event.
hope this helps.