According to the Gemara you quoted it asks this very valid question. It says that רב שמן בר אבא says that the word "כימים" is for generations ("דורות"). This means according to Rashi, that just like this generation made the 14th and 15th different based off of walled cities, so too following generations should do the same.
I suppose you might be asking why doesn't רב שמן בר אבא decide to darshan entirely like ר׳׳ש בר נחמני? First of all because he is bothered by the different way of writing זמן זמנם זמניהם. But also at a quick glance it would appear to me that ימים fits well with the verse before it to be reflexive of 14/15 just mentioned, perhaps רב שמן בר אבא understood the word "כ" to be "just as" (ie: to make the drasha as close as possible), so it works well for generations as it makes 14/15th just as making then, instead of the other opinion that understood just as 14/15th so too 11/12. צ׳׳ע