[NTG-context] a ghost in the columns ?

Hi, please have a look at the minimal below (tex+pdf): We always have a kind of heading followed by a block of two columns. But suddenly on page 6, entry "Cassius Dio" has a much larger vertical space above than "Novellen" for example! Why? And how can this be avoided? Any help very much appreciated! Steffen (MkIV) -------