Hi there,
there could be different reasons for this behaviour...
as MinusZero said, it could be any javascript script in initialize; docready; or anywhere that hold script which would add instances to your subform.
In the other hand, sometimes it's a problem with the software when reading the file. If you have subforms in different places in the form with the same names, it could happen that the software adds a random instance to one of them. By giving them a distinct name it can fix the problem.
Hope this help.