Two people with identical reflexes can get noticeably different scores on an online reaction test. The reason is that the number you see is not only your nervous system. It is your nervous system plus every piece of hardware and software between the signal and your tap. Knowing where that extra time hides helps you read your score more fairly.
Imagine a relay race where your body runs only one leg. Before you even start, the screen has to draw the green colour. After you tap, the device has to notice the touch and pass it to the browser. Each handoff adds a little time, and the test can only measure the total.
A screen updates in fixed steps called frames. A standard 60 Hz display shows a new frame about every 16.7 milliseconds, so the green may appear up to one frame after the test asked for it. A 120 Hz screen halves that step to about 8.3 ms, and a 240 Hz gaming monitor to about 4.2 ms. Some displays also process the image before showing it, adding a few more milliseconds.
Touchscreens scan for fingers many times per second and then filter the signal so that accidental brushes are ignored. That work takes time, and it differs a lot between phone models. Wired mice usually report clicks very quickly; some wireless and Bluetooth devices add more delay.
A busy phone, a low battery saving mode or many open tabs can make the browser slower to react. Most of the time the effect is small, but it can make results less consistent from one try to the next.
Rule of thumb: the same person can score a few dozen milliseconds faster on a fast desktop setup than on an older phone. When you compare your score with a friend's, compare on the same device if you can.
Partly, yes. Countries where more people play on fast computers may get a small advantage, while countries where most people play on budget phones may look a little slower. That is one reason the BlinkCup ranking is a fun comparison rather than a scientific measurement. Averaging across many players helps, because each country includes a mix of devices, but it does not remove device differences entirely.
Curious what a "good" score looks like? Read our reaction time guide.