[SalesForce] SOAP RetrieveRequest returns internal Data Extension names

I've raised this support ticket with ExactTarget Support, but they have been to assist so I am posting my question here.

I am using a SOAP RetrieveRequest (with a Retrieve SOAPAction) to get all data extension names using the following envelope:

<?xml version="1.0" encoding="utf-8"?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <soapenv:Header>
        <wsse:Security soapenv:mustUnderstand="1" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">
            <wsse:UsernameToken wsu:Id="UsernameToken-32259181" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
                <wsse:Username>username</wsse:Username>
                <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">password</wsse:Password>
            </wsse:UsernameToken>
        </wsse:Security>
    </soapenv:Header>
    <soapenv:Body>
        <RetrieveRequestMsg xmlns="http://exacttarget.com/wsdl/partnerAPI">
            <RetrieveRequest>
                <ObjectType>DataExtension</ObjectType>
                <Properties>CustomerKey</Properties>
                <Properties>Name</Properties>
            </RetrieveRequest>
        </RetrieveRequestMsg>
    </soapenv:Body>
</soapenv:Envelope>

This works as expected, but the SOAP response returns several other names that don't appear in my ExactTarget MC dashboard:

• _MobileAddress
• _MobileSubscription
• _PushAddress
• _PushTag
• first list
• second list
• qwerqwer

I've been informed that these are "internal extensions".

Surely this is a bug? Why would these internal DE's be returned? Is there a way of using a filter to exclude these internal extensions in the SOAP response? Perhaps you can use regex in filters, but I'm not sure.