When you export an artboard, the artboard size increases by one pixel if the artboard X and/or Y position includes a fractional (non-integer) value.
For example, consider you have an artboard of size 600 x 200 and it is placed at the following X and Y coordinates:
X: 298.28 pt
Y: 207.31 pt
Now, when you export this artboard by choosing File > Export > Export for Screens, the dimensions of the exported artboard increase by one pixel (601 x 201) to take care of the decimal points.