First off, there is no difference between the two signature field objects.
I'll do my best to explain why there is a difference in how the signature objects are named in the Signature pane of Reader\Acrobat. When you add the signature field using LC Designer, the object is defined in the underlying XML (XFA) that defines the form. The (SOM expression) you see in Reader (via the signature pane) matches the structure of the form.
When you add a signature field to a form using LiveCycle Digital Signatures ES2, you are appending a signature field "on top" of a PDF form (the underlying XML has been rendered into PDF). The signature field is not defined in the XML, therefore Reader\Acrobat do not display the same naming syntax.
Does this help?
Thanks for the answer, Steve.
Is there a chance to hide the SOM expression of the fields created in designer and to present only the field name?
I am not aware of anyway to modify or affect how Acrobat\Reader displays the naming convention in the signatures pane.