graphics – Export a graph to pdf but size does not match

The following is a minimal example that I intended to obtain a graph of image size 3cm:

cm = 72/2.54;
r = RegionPlot[x^2 + y^2 < 1, {x, -1, 1}, {y, -1, 1}, Frame -> True, 
  PlotRange -> {{-1, 1}, {-1, 1}}, FrameLabel -> {x, y}]
Export["test1.pdf", Show[r, ImageSize -> 3 cm]]

After opening test1.pdf by Adobe Acrobat reader, I found out that the real size of the pdf file is 2.19 * 1.98 cm. Why does not the size match, and how can I resolve this problem?