I wanted to say that it is because I thought that I distinctly recall there being a year where the league year began on March 3rd or 4th, but perhaps I'm confusing start of league year with start of free agency.
You aren't. The league year always starts March 1 and free agency did start about March 5th or so. I think it was moved back a week or so to allow teams more time to to sign their players.
When it was moved back, I couldn't say.