What's wrong with this picture?

(or, at least, highly suspicious--and why?):

from  Morioka et al., arXiv:0807.0101, PASJ in press.  Black dots are H-alpha emitting galaxy candidates (via a narrowband filter selection), grey regions are masked-out bright stars.  The authors use this sample to compute the clustering and luminosity function of star-forming galaxies at z=0.24, and note that the clustering in this field is stronger than in the COSMOS field.  First one to post the right answer in comments wins a beer at the next borrel.