when does ribbon's dropdown ctl's getItemCount 1st get called

[scw-tzg]

Word 2007, VS2008,VSTO

I have a custom ribbon that has dropdown control with getItemCount callback
defined:
<dropDown id="tcsSetowner"
onAction="OwnerDropdown_Click"
imageMso="FileManageMenu"
label="Set Owner"
screentip="Select a TCS Owner system for subsequent activity."
getItemCount="GetOwnrItemCount"
getItemID="GetOwnrItemID"
getItemLabel="GetOwnrItemLabel"
getSelectedItemID="GetSelectedID"/>

When would that callback first get called? During ribbon's constructor?

On one machine where my solution is installed, it is called during
constructor; on another machine, it 1st gets called sometime later.