Well, to reach someone's achilles tendon (which is in the back of the ankle) you would have to be able to reach their foot. That would mean, either you have a long sword (which Romans didn't) or you stoop/bend over (bad idea usually) OR they have their foot somehow within your reach. Now, if you mean cutting the tendons in the back of the knees *that* makes a bit more sense. Far easier to reach, every bit as debilitating (if not more so).
I suspect this is the case as I can see absolutely no connection between the ankle and the term "hamstring" (which, I believe, is actually a muscle going up the back of the thigh).