Kurigin cannot fully comprehend Goku's strength or Nappa's or Vegeta's. Hence why he believes their power to be beyond their imagination. Additionally, all of Nappa's durability feats tie in line with this. From Kuririn's blast, to Chaozu's explosion to Gohan's Masenko, time and time again he was completely unfazed from anything when someone of his strength level, should have been. Thus making him appear stronger then imagination, and hence why Goku was so surprised at his toughness, but continued to mock his strength.
There's nothing contradictory here. Anything else?