Some ovulation kits also base the "fertile time" on rising estrogen levels.

http://www.babycenter.com/404_can-you-get-pregnant-during-your-period_1460117.bc

Some ovulation kits also base the "fertile time" on rising estrogen levels rather than the peak of luteinizing hormone, or LH, which is the actual trigger for egg release. In those situations the "fertile time" may seem to be longer.