According to Hofstadter... (Score 5, Informative)

On page 717 in Godel, Escher, Bach, Hofstadter explains the "central blemish" as follows...

"Though the blemish seems like a defect, perhaps the defect lies in our expectations, for in fact Escher could not have completed that portion of the pircture without being inconsistent with the rules by which he was drawing the picture. The center of the whorl is -- and must be -- incomplete. Escher could have made it arbitrarily small, but he could not have gotten rid of it."

What Lenstra was able to do was to figure out the structure of the picture. From there, he was able to generate a suitable center so that none of the relationships between the four various pieces are disrupted.

This is the reason why this is some pretty neat work.