I suppose that I would directly clarify with your dad's oncologist to confirm the current survival percentages for treatment/non-treatment groups - I bet that you're right about the math situation, and the doctor was trying to make it simpler by referring to people rather than percentages, but if those numbers are going to significantly influence decision making, they should be explicitly clarified by the expert.
also, I would imagine that histopathology is being performed on the tumor; this may help to further identify the risk of metastasis (based on the appearance of the cancerous cells) that is not yet grossly appreciable; if in fact there is stage IIIa colon cancer, it can involve up to 3 lymph nodes OR be invasive through the layers of the bowel wall up to the tissue surrounding the lymph nodes (but not yet involving the nodes themselves).
there is a wealth of good information available online about this type of cancer that can be pretty helpful - the NIH website is pretty informative without being overwhelming:
http://www.cancer.gov/cancertopics/pdq/treatment/colon/Patient/page2#Keypoint12
also, there are several pretty recent articles available about the benefits of chemotherapy for this stage of tumor; I've only skimmed the following manuscripts (which are targeted at medical professionals, be forewarned!) but this one suggests that chemotherapy is less often pursued (for whatever reason) in older patients or those with other concurrent disease, though within their (arguably limited) study population, patients receiving chemotherapy were likely to live longer:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989633/?tool=pubmed. another (abstract only, not a full article) discusses that some of the newer chemotherapy regimens for stage 3 colon cancer may not confer as much benefit in patients >70 years old than in those <70 years old:
http://www.asco.org/ascov2/Meetings/Abstracts?&vmview=abst_detail_view&confID=65&abstractID=34878. however, your dad's oncologist will have the most recent information about all of these options!
sorry for the wall of text - good luck with everything!
disclaimer: I'm neither an oncologist nor a physician (I'm a veterinarian!)