Since you purposely insist on acting like an ignorant sheep, i'll explain it to you this way:
Many shonen magazine have tried to tap into the seinen market, and vice-versa. Jump also has shojo, for example, even though the magazine has shonen in the title. There are genre-like classifications for what usually classifies something as shonen, seinen, josei, shojo, etc.
But just because it is published in Jump doesn't make it shonen. It means they probably picked it up because it began as shonen, it had enough similarities to shonen titles, or because they think it would appeal to shonen fans. In the case of HxH and several other manga, it began as shonen, but isn't really a shonenanymore.
The manga is only in weekly shonen jump because the autor got a great deal, otherwise it would be straight seinen, a long story about a boy and his "friends" on adventures on a twisted sick ill world, just like ours.
It's a seinen, disguised as a shonen. You could also call it a seinen-shonen hybrid or soft-seinen, for short.