At some schools, not everyone gets the same money! Some fellowships pay more than others.
However, at many schools, everyone does get the same money but they come from different sources. Something named like the Dean's Fellowship would indicate that this is money coming from the University, not the department. It's never clear if the University is providing 100% of the money or if they provide 50% and the department provides 50% etc. Sometimes you get something like a "Department Fellowship" which indicates the money is internal to the department, or a fellowship with someone's name, which might indicate that person donated some money to either the school or the department with the purpose of funding students.
The wording in this email seems to suggest that each department might get 1 or 2 Dean's Fellowships. This might mean that the University provides some money towards each department to help them pay for students and this automatically goes to the top 1 or 2. Or, perhaps the University is able to fund the top 10-20 applicants from across the whole school, and each department gets to nominate their top candidate(s). If their candidate wins, it's really a win for the department since they save money on you, allowing them to spend it on recruiting an extra student, perhaps.
In any case, congratulations on the honour! I wouldn't worry too much about the names of the fellowship. Grad student funding usually comes from a variety of sources and the names just vaguely reflect where it came from.