From 1016 to 1030 the Normans were pure mercenaries, serving either Byzantines or Lombards, and then Sergius of Naples, by installing their leader Rainulf in the fortress of Aversa in 1030, gave them their first pied-à-terre and they began an organized conquest of the land.
For a 28 year period the Normans were pure mercenaries.
The Normans were pure mercenaries from 1016 to 1030 which was only 14 or 15 years, not 28.  The system likely had trouble with the math computations.
( z1 mercenary :ARG1-of ( z2 pure ) :domain ( z3 person :mod ( z4 country :name ( z5 name :op "Normans" ) ) ) :duration ( z6 temporal-quantity :quant 28 :unit ( z7 year ) ) )