Then they would be closer to the fastest horse in there race than any of the horses in the 6th race. Think of it as a calibration step so you can find the positions of every horse by comparing how close they where to the finish line with how close the winner was to the finish line in the final race. (adjusting for a fractionally shorter race as in you don't know it took an minute or an hour but with 98% of the time they got to x so if they had been in the 6th race they would have ended up at y.)
PS: I am treating this as a real world trick question, if your talking about a virtual world where the only information is a horses rank then yea you would need a 7th race with 5 horses assuming no ties.
