Percentage noun – A measure of how often an event will occur instead of another. Advantage and percentage are semantically related. Sometimes you can use "Advantage" instead a noun "Percentage". Nearby Word: percent
This is also the reason why playing a high number of low-stakes games in Casinos increases the chances of the house making money; the house advantage only affects who wins a small percentage of the time, but this advantage "evens out" over the long haul.
That's how outgunned and outnumbered humanity was during the war; even early in the war, when the Covenant was dedicating such a small percentage of its forces that humanity actually held the numerical advantage in space, the UNSC was still forced on the defensive.