I don't think I have to explain (though the first explanation that comes to mind is that you were familiar enough with the location to have a pretty good idea of approximately which direction you were facing).
The point is that magnetic field lines are a convenient diagrammatic tool, but don't have a direct correspondence to any physical reality. It's just like the way we sometimes draw electromagnetic radiation as a sine wave, but try as you might you won't ever observe little squiggly sine waves in physical reality.
You can point to points in space and say that the scalar value of their vector field is the same or different, and say whether or not there is a continuous path of scalar value sameness between them.
The iron filings actually concentrate the field, which leads to a bit of an avalanche effect as the concentrated field attracts more filings towards it that creates the lines.
The point is that magnetic field lines are a convenient diagrammatic tool, but don't have a direct correspondence to any physical reality. It's just like the way we sometimes draw electromagnetic radiation as a sine wave, but try as you might you won't ever observe little squiggly sine waves in physical reality.