Into the document library, you can upload some document or content , which gets stored in the sharepoint content database.
A list on the other hand is just a piece of data, just like SQL Table, although it is possible to store binary content into list as well using attachments.
When submitting with Infopath Form Services, the submitted data is generated as XML which refers the original form template (.XSN). This XML file can only be stored in document library by infopath and hence the troubleshooting step.
FROM: IDEAMSINFOTECH
MAILID:info@idreamsinfotech.com
WEBSITE: http://www.idreamsprojects.in/
MBNO: 7200004613