I would guess there's an issue with using dual-monitors with different refresh rates.
The timing project showed times of 8.1 and 9.6 seconds, a ratio of about 1.185. The ratio of 75 Hz to 60 Hz is 1.25. It's not super close but the right ballpark, so I'd guess in this specific setup it's possible it ends up using frame times from a different display using a different refresh rate, resulting in the wrong speed. I'd speculate that if it was using frame times from the 75 Hz display (13.3ms frames) on the 60 Hz display (16.7ms frames), it would run slightly too slow.
I'm sure this is not an issue with Construct since it will always work at the same speed providing it gets the right frame time measurements. I guess it's a Chrome bug, but it's difficult to see how this can be resolved - e.g. if you have the window displaying half-and-half on each display, what framerate should it run at? It's a weird setup and hard to know what it ought to do in that case.