If you measure the time the swimmers take very precisely and thus can tell that one swimmer reached the target a few milliseconds earlier, that could be because they are a faster swimmer than the second-placed. Or they could just have had a shorter lane to swim and win despite being slightly slower than the other. They swim around 2.39 mm per ms, and lane differences are allowed to be up to 3 cm, so this can make ~12 ms difference where you don't know where it is from.