An Ork warboss is sitting in a trench with his horde of boyz behind him, when he hears an Astartes shout from the other side of no-man's land "One Space Marine is worth ten Orks!" Recognizing a challenge when he hears one, the Warboss immediately sends ten eager boyz up and over to make the space marine eat his words. A moment later, the sounds of battle echo across the field, and ends just as quickly without a single Ork returning.
Then the same voice calls out "One Space Marine is worth fifty Orks!" Now a bit miffed, The Boss sends fifty lads to stomp in the challenger's face. Once again, the sounds of combat are heard, the Orkish warcries filling the air, but after a few minutes it dies down and the greenskin attack is completely silenced.
"One Space Marine is worth a HUNDRED Orks!" cries the champion. Utterly furious, the Warboss summons a hundred of his best warriors, taking a while and enlisting a few Grots to help count that high with their fingers, and the warhost charges the Imperial position with a deafening WAAAGH. An epic battle is heard for ten minutes, but much to the boss's frustration, even that peters out until the field is silent once again.
However, this time a single nob crawls back into the Ork trench, and tells him "They cheated, boss! Dere was TWO of 'em!"