Unexpected function evaluation result in Plot

Question

I find some strange behaviors of Plot. The code is:

f[x_] := (Print[x]; x);
Plot[f[a], {a, 1, 3}, MaxRecursion -> 0, PlotPoints -> 2, 
 EvaluationMonitor :> Print[{a, f[a]}]]

And I get the output:

1
a
1.002
1.
1.
{1.,1.}
3.
3.
{3.,3.}

I understand that

1.
1.
{1.,1.}
3.
3.
{3.,3.}

are normal outputs as expected. But what are

1
a
1.002

before that?