How can cancer be gone if the whole tumor wasn't removed after surgery?

Hi, I haven’t posted in a while, well I have some great news; I had my colectomy on December 11 it went well, I’m still recovering. According to my surgeon they were not able to remove the whole tumor but according to her there’s no more cancer present. So I don’t understand if they didn’t remove the whole tumor how come the cancer is gone. Well I’m going for a CT scan on feb 2,to be sure the cancer is gone and see what’s the next step. I might still do another three month of chemo. In the mean time I am back at work it feels great to be back in the real world, I’m so grateful for all the support I got from everyone in this site it really helped me during the most difficult times of my entire life.