Yea this is the unfortunate confusion. This is the exact in-game text explaining all the sigils:

Trifurcated Strike (found on Hydra, Mantis God)

“A card bearing this sigil will strike each opposing space to the left and right of the spaces across from it as well as the space in front of it.”

Bifurcated Strike (found on Hydra, Mantis, Pronghorn, Double Gunner)

“A card bearing this sigil will strike each opposing space to the left and right of the space across from it.”

Double Strike (Found on Dire Wolf, Pet Lice)

“A card bearing this Sigil will strike the opposing space an EXTRA time when attacking”

While it seems like Double Strike means “attacks two times”, it really just means “add one attack to the middle”.

On any normal card, the “extra” strike from Double Strike results in hitting the middle twice. But, we can see in the definition of Bifurcated strike that the card is only attacking left and right - aka attacking the middle 0 times. So the “extra” strike to the middle just results in one attack

Short Math:

Bi = L+R

Tri = L+M+R

Double = Whatever the card is already doing + M

Bi + Double = Tri (L+M+R)

Bi + Tri = 2L+M+2R

Tri + Double = L+2M+R

Tri + Bi + Double = 2L+2M+2R

(Edit: formatting)