Relax. The margin of error is probably small enough as to not fret about it. But there is a margin of error, and Dave "Voluntary Xchange" Tufte researched it in the use of timers in swimming.
[I]n swimming there are a lot of ties.There's a more intriguing recent rule change in swimming. In the past few Olympics, I've noticed racers coming off the blocks or the turns and making like an eel rather than surfacing and starting to flail immediately. And there are apparently rules governing the eeling zone (dear reader, if you know the technical terms and the rule references, please advise) that include designating it with different colors on the lane dividers (yet another margin of error?) And it appears that the eeling zones are different for sprints than they are for the distance races, e.g. the 1500 meters. Or is it as simple as, at the margin, a small advantage from staying submerged matters more in the shorter races?
How is it possible that there are ties when we can measure time accurately to 3 to 6 digits?
Here’s why: because in 0.001 seconds a swimmer travels about 2 millimeters, but the construction tolerance for differences in pool lane lengths is plus or minus 30 millimeters (due to expansion and contraction from heat and cold).
FINA (which governs international swimming) recognized 40 years ago that improvements in timekeeping simply is not differentiating swimmers better.