Thank God It's Friday... Again - S1-E6
Other mistake: The planet population works in the fields, harvesting large brown biconal fruit. They seem to be heavy, as a bag containing about three of them is a burden to the person carrying it. When the leader of the planet shows one to Crighton, she picks one up with three fingers.
Chosen answer: It's never explained. John mentions a theory that his less technologically developed craft simply interferes with the wormhole less than the "superior" Vipers, but they never make a concrete determination on that point. But we also have to take into account the fact that Moya traverses wormholes with no ill effect as well.
Garlonuss ★