The short answer is that, in a lot of Universities, the DGS has less control over outcomes than you might think. Some Departments get a pot of money from their Graduate Schools and can decide on admission and allocation simultaneously. Others, like ours, can make decisions about admissions but only have a limited ability to control funding allocations. In our case, we enter every one of our applicants to an internal, Graduate School-wide fellowship competition, and we have precisely zero control over who wins how much. We have a bit of funding to even out the gaps, but surprisingly little. When we allocate the funding that we have, we have to make sure that we haven't overlooked one of the Graduate School's rules for how it can be allocated and to whom.
The finer points are really, really tedious. But the upshot is that what you see as an intentional decision to notify some people at one time and other people at another time may or may not be intentional. In my experience, it's more likely to be the result of a tangled bureaucracy, or of overworked people who can only devote sporadic attention to finalizing admissions decisions, or both.
Unfortunately, that cuts both ways: I can't really take credit for being humane, since the Graduate School sends out the admissions letters and the funding letters in two simultaneous waves.