All 400^k possibilities are very very likely to have less cumulative difficulty than the main chain, because 75% < 100%
You're not appreciating how the statistics work out here.
75% being less than 100% just means that the delay distribution is shifted to the right.
But sampling from a huge number like 400^k means you'll get much further into the left-tail
of this shifted distribution, so you can easily beat the unshifted average.