I would check to make sure that the nic drivers on the laptops were current (updated); check the laptops advanced nic settings and make sure they are autosensing to 100MB and full duplex; make sure you are using known good cat5 cables with the laptops and in addition make sure you are testing apples to apples (same time of day, same sites, etc.) Cable modem bandwidth is "shared" and can vary based upon number of users, upstream traffic patterns, etc.
Also might want to try some of the other "speedtest" sites:
Speed Test : upload and download | DSLReports.com, ISP Information