Don't worry! Your app is working fine 😉 Here's what happens:
1. Your App logs all exchanges with nearby TraceTogether devices, i.e. TraceTogether apps and tokens. Because Bluetooth's range is about 10 metres (and could be more), phones close to you such as your neighbours' could exchange signals with your phone, sometimes through the walls/ceiling/floor. But not to worry, because...
2. When a patient confirmed with COVID-19 uploads their Bluetooth proximity data to MOH, it is processed and filtered based on duration and signal strength to identify only the close contacts.
Filtering is not done on the phone because it's not a straightforward cut-off. The algorithm calibrates the data to account for different transmission powers of different phone models.
Then what do the two numbers in my app mean? What is the difference between the number of devices nearby, and the total number of exchanges today?
The number range shown in your App (e.g. 11-15) shows the number of unique TraceTogether devices that exchanged Bluetooth signals with your App in the past few minutes (imagine it’s the number of devices that your phone has "talked" to in the past few minutes). This number may differ between phones models due to different transmission powers.
The number below it (e.g. 10,000) shows the total number of Bluetooth exchanges between your App and the other TraceTogether devices, in the day (imagine it’s the total number of times that your phone talks to other devices in the day). Each pair of devices often exchange more than one signal. The longer the devices are in one another's Bluetooth range, the more signals they exchange. And the counter resets everyday.
You might be wondering why we don't show the total number of unique TraceTogether devices that exchanged signals with your App throughout the day — something that's much easier to understand. We agree :) However, we value data privacy as much as you do. TraceTogether is designed to prevent malicious actors from tracking individual users over time, and we do that by frequently changing the 'nickname' that your App exchanges with other TraceTogether devices. The key to match these 'nicknames' back to a user stays in a secure server, only used when a COVID-19 positive patient uploads their TraceTogether data. This is why the number of TraceTogether devices nearby shown in your App is only an estimate, because each unique TraceTogether device could appear as multiple 'nicknames'.
P.S. You may have noticed that the total Bluetooth exchange counts increased significantly after the App update in Sep 2020. This is because we improved the way your App communicates with the newly launched TraceTogether Tokens.