I am at a loss to understand how the station can be busy when so few are recorded as using it. Is there any reason you can think of why this apparent contradiction exists? Is it that a few peak trains are busy and the rest unused at Salford Central or is some other factor at work?
Someone more knowledgeable may be able to confirm, but it might be that a lot of people who use the station have "Manchester Stations" tickets meaning that the actual number of people using it is under-recorded. (a similar situation occured passenger usage figures at Wakefield Kirkgate station until recently).