I would expect [count]% to jump back and forth between matching braces count times or something like that, but instead, typing that command seems to move the cursor to a random place in the buffer.  It seems to always go to the same place for constant count, but different places for different count. So what exactly does that command do?
What does [count]% do?
cursor movement
From :help N%:Go to {count} percentage in the file, on the first non-blank in the line |linewise|.  To compute the new line number this formula is used:  ({count} * number-of-lines + 99) / 100  See also 'startofline' option.I didn't know this either, by the way. But I typed :help %<C-d>, and N% looked like a good match ;-)Also see: How do I navigate to topics in Vim's documentation?.