The score goes from 0 to 140 so that there's some room for when the computers will get faster.
When we started working on this, all browsers were maxed at 140, so the computation got changed.
I thought the front page goes to 140 just because it is modeled after actual GM dashboard speedometers produced ~1960-1990, sometimes having range 0-85 MPH, or 0-140km/h in metric markets.
The speedometer graphic was inherited from Speedometer 2. When Speedometer 2 was released, scores were in a reasonable car-speed range. The combination of hardware and software improvements meant that early versions of Speedometer 3 (which includes a subset of Speedometer 2 tests) were consistently scoring above 140, so we adjusted the scaling factor (IIRC, by ~20x) to give plenty of room for future improvements.
Nothing actually stops the score from going higher than 140, it will just max out the visual dashboard at that point. On Speedometer 2, Safari on M3 Macs ended up over 500. At scores that high it’s harder to have intuition, thus the changed scale of the new test.