I think the reason for which the 25 point buy is lower than 4d6 drop lowest in terms of total modifiers is that by using point buy you have an inherent (not too small) advantage in being able to choose exactly your stats. It allows you to plan exactly what will happen at 4th and 8th level when you'll get those free points, as well as requirements for feats and prestige classes, in the most efficient way, while with the random generation you risk being screwed and you'll likely waste some points. It's the same reason for which the fixed HP optional rule in the DMG suggests giving half max hit dice instead of average (eg, 6 for a barb instead of 6.5). Safety is an advantage in itself.