@LLXC This has been a thing for a while. From what I understand, it is an issue with the interpolation. As you zoom in, the interpolation becomes a slightly higher resolution. I think it is a minor issue and don't think much of it. I've only noticed a difference of one or two units as I zoom in so it's not that big of a deal for me.
What I would recommend is zooming in as much as you like to get more accurate forecast readings.