There is a common misperception in the running industry that RFID chip timing is completely accurate and foolproof. The first thing that you have to remember is that the RF means Radio Frequency which is simply radio waves. Radio waves are affected by water, solar flares, metal, and other RF interference. It is why you sometimes drop a cell-phone signal or can't even get a signal sometimes. It's also why you sometimes can't get your favorite radio station in an area that you normally don't have any problem with. The same situations that affect cell signals and radio also affect chip timing.
One of the most exasperating issues that timers everywhere face is having to do with failed chips. If you ever get a group of timers together to talk about their biggest problem, the top of the list will be read rates. Everyone regardless of the equipment used and chip type suffers from the same ailment. There is no perfect solution and timers spend a lot of time and energy trying to figure out the optimum solution to the problem.
It's important to understand the technology a little bit to see why it is such a big issue. Most timing systems use what is called passive RFID. What this means is that the chip tha the runner is wearing is not actually active until it detects the RF from the timing system. This RF is used to power the chip so that it can communicate with the system. The chip powers up and communicates in short bursts its identification. This happens in a very very small amount of time. This communication is by radio wave.
So what can go wrong? One is that the chip can be programmed correctly but simply fail to activate at the finish line. This can happen because it doesn't detect the RF or the signal can actually get blocked. Wet weather is one factor. Other RF interference in the area can also be a factor. The RF is really line of sight so people using the chip incorrectly can cause an issue such as wearing a chip bib on their back instead of front, or in the case of an ankle chip on their inside instead of outside leg.
There are several things that timers use to try to mitigate the problem. The costliest but most reliable is to simply add more systems. In the case of antennas it means adding additional antenna arrays. In the case of ground mats, it means adding additional mats lined up so that the runner has a longer stretch of possible reads. What runners don't realize though is that those additional systems are really hiding the problem rather than solving it. The timer sets an offset for the additional mats/antennas so that it appears that their time is accurate when in fact it is not. If for example, there are 4 mats in front of the finish line and you only get read at the furthest mat, your chip might have been read 20 feet away from the finish line and never again. The timer will have set an offset of 4 or 5 seconds to account for the extra distance. That offset will be added to the time read so that your finish time looks right but in fact no one really knows.
Average failure rate of chips in the industry is around 2%. That is not to say that every race will miss 2% of its participants but on average a timer can expect to miss that number with a single system. By adding additional hardware or manual backups, the timer can get much closer to 0 failures. The problem is one of cost. Timing equipment is expensive and the timer has to coordinate with the event to determine how to handle that. A timer needs to recover the additional cost in terms of manpower and equipment but the event is oftentimes not willing to pay for it. It's a trade-off and is determined by size of the event, type of event, etc. Every timer I know goes through that calculation to determine the best way to handle it. None of us like to miss a time but especially with large events, we're dependent on the technology and the physics of it all makes it tough. If you discover that your time was missed, talk to the timer and let them know. They are happy to work with you to correct it.
See you at the race!