That's not entirely true. Sure, the average is closer to your soul level. But, you can invade up, and by a significant amount and often. And, no they were not hacking. Here's my proof: https://soulswiki.forumotion.com/t22160-forest-invasion-sl-rangeshey its andres wrote:
Also, you will never fight someone "5x your SL" in the forest. I know that this is an exaggeration, but forest invasions are the same as summons signs so if someone is too powerful or gives you an unusual amount of souls, then they are hacking.
And, please do not jump to the generalization that someone is hacking without absolute proof (such as a never ending lifebar or fast rolling with armor too heavy even with maxed endurance.
Here's my last opinion on the forest. As a catbro, you are charged with defending the forest against the trespassers. All of your forest buddies (the NPCs, sorcerer, mace, knight, etc) attack on site. It is reasonable to assume that you were meant to do the same. That was the game's intent; how it was intended to be played. You are in a war against the trespassers. IMO, it is dishonorable to not treat it as such. (That doesn't mean you have to be a dink after your victories by taunting though.)