You have already answered the question posed...the results weren't the same were they? :)
plot is a "high level" and line is a "low(er) level" function; with plot many behind the scenes details are done when the function is called while line simply puts the points on the existing axis.
At minimum,
hold(ax1,'on')
first or use the named property 'NextPlot' with plot as 'NextPlot','Add'