[Zope] Script (python) shows transaction for adding folder

I have a script that accepts an html form entry (submitted from a dtml-document) and tries to do various file uploads into the zodb (actually, they are ExtImages). All of this works (pretty well), but in testing, I've noticed that each time my script performs this form validation/submission process, there is a new transaction present in the undo list. The transaction refers to adding a folder (and probably another one to add the actual image) to the zodb for the upload. So, my question is, why does the transaction appear in the undo list for the script, and not the parent folder I'm adding to (which is a BTree folder if that makes any difference)? I guess it's not a problem, just seems a bit odd to me. thanks tim