This not a bug per se... it's actually in the category of "how things work".
This is a font cache issue and is purely a display artifact. You will see it more prominent or less prominent depending on the resolution of your screen, and your current view percentage.
Once a character of a font is rendered for the screen, each is stored as a bitmap in the cache for a particular size and orientation, and reused each time to speed up display avoiding having to render them again. Especially on pages with lots of text, not having to re-render each character saves time in redrawing the screen. However, because the bitmaps are already rendered, they can only be placed according to the pixel grid of your screen at that view, and will "snap" to the next available pixel column based on its position. When you kern like you wwre doing, you are moving the characters ever so slightly, but oOnce it gets past the half-way of a pixel's distance away, it snaps to the next available pixel position. And since each letter is defined with a slightly different starting position, the "snapping" will happen randomly as required in a line of text. The more you zoom in, or if you have a high-pitch monitor like a Mac's Retina screen, the less you will see this issue, or if you use a high-pitch display. This does NOT affect output, although the same thing technically happens on a high-resolution output device, but at such a high resolution that a one-pixel difference will not matter.
The font cache approcah has been in use since the very beginning and is the basis of PostScript printing
Other programs may not use a similar font cache approach, especially at certain type sizes and resolutions. The sample you saw with much smaller type and very low-resolution is rendering each character repeatedly and anti-aliasing them on the fly, probably since there is not much overhead there and it become beneficial to do it that way.
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