It's not the number of elements that dictates rendering - it's their (relative) size.
Here is an example. Let's say you have an element that is 16px at 100% zoom. That means when you zoom out to 200%, that element becomes 8px. As you continue to zoom out, that element gets smaller and smaller, until it gets to be ~1px. It can actually dip under 1px if you continue to zoom out, and that's where things can get weird.
If an element dips below 1px, Mockups rounds it's size up or down. A couple of things influence this (where the viewport is, the actual size of the element, etc). If it rounds down, then it doesn't render the element. If it rounds up, it gets rendered at 1px.
In your data grid, some of the lines are being rounded down, so Mockups doesn't render them at that zoom level. They are still there, and you will see them if you zoom in, but they are too small for Mockups to display at that zoom. This isn't uniform because Mockups takes other things into account when deciding to render an element. That's why the rendering seems random.
Does that make sense, @DebK?