Blank zone with imshow()

When using imshow(), why does there always seem to be a blank zone along the southern and eastern edges of the figure? For instance:

X = rand(10,10)
imshow(X)

plots a luminance image of X, which seems fine, except for the lower and rightmost edges, which are blank. I may be misunderstanding the purpose of imshow, but skimming through the code didn't give me an answer. I am using matplotlib 0.52 on WinXP with either GTKAgg or TkAgg.

Thanks !
Dominique