Ok, your first mistake is stating that an 1800 billable requirement = working 7.2 a day and 1900 = 7.6 hours a day. To bill those hours, you generally have to be in the office several hours longer because you are constantly doing things that are not billable. Also, many firms have additionally hours they require for professional development that are on top of the billable requirement and can amount to several hundred or more.
Finally, you can't control when the work is in. So if there is a slow month, and there always is at a law firm, then you have to play catch up.
So you say you are "going for secondary markets" and then say you are working at two BIGLAW firms this summer. Are you a summer associate? http://www.law.yale.edu/documents/pdf/CDO_Public/cdo-billable_hour.pdf
Check out that link as well.