Problem with countour plots - double lines around minimum area

Hi, I am having a problem with contour plots. I have a 2D map of fuel consumption. x-axis is RPM, y-axis is BMEP. The map is highest around the edges, and lowest in the middle. Since it is composed of discrete levels (i.e. the values in the map have been rounded to known levels) and I want the contours to appear around those levels, I am using the following command to plot it:

contour(RPM, BMEP, map, unique(map));

This seems to work in some cases, but in others a double line appears around the inner most area (the lowest value on the map). I have simplified as much as I can below to show what I mean:

map =
 600   600   600   600   600   600   600   600
 600   600   600   600   600   600   600   600
 600   600   600   600   600   600   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   200   200   200   200   600   600
 600   600   600   600   600   600   600   600
 600   600   600   600   600   600   600   600
 600   600   600   600   600   600   600   600
figure;
contour(1000:500:4500, 0:18, map2, unique(map2))

What I get: http://i.imgur.com/ZP285.png

What I expected to get: http://i.imgur.com/azY0j.png

I'd expect the contour at 600 to surround the entire plot, not to be placed on the same boundary as the contour at 200.

The double contour does not appear if there is a low value region surrounding the values (e.g. if all the 600s are replaced by zero in the above example).

Can someone explain what I'm doing wrong? Thanks.