An important paragraph from p. 174 says:
"The randomization/permutation test and the bootstrap test were introduced in the previous two chapters as methods to test for group differences. Which should be used? From a statistical theory point of view, the difference between the two methods is that the randomization method is conditioned on the marginal distribution under the null hypothesis. This means each permutation of the data will have the same marginal distribution. *The bootstrap method allows the marginal distribution to vary, meaning the marginal distribution changes with each replicate data set.* If repeated samples were drawn from a larger population, variation would be expected in the marginal distribution, even under the null hypothesis of no difference. The variation in the marginal distribution is not expected; however, there is only one sample from which groups are being randomly assigned so long as the null hypothesis is true. Thus the choice of the method comes down to whether one should condition on the marginal distribution or not".
Can someone (authors?) please explain to me what the highlighted sentence means? What multiple marginal distributions are there?