A Series A round title of the round may also have been required because a new share structure was being created to satisfy the investors requirements given the larger amount of money they were investing. The more money, the greater the perceived risk. The new share structure may have provided a greater reward or multiple in the event of an exit or public offering, as well as created new 'preferences' relative to the common shares likely already in place from the true start up phase (or angel round).

To understand more about series funding from series seed to series E check this out: https://www.startups.com/library/expert-advice/series-funding-a-b-c-d-e