because that series is not accurately rated.
It is very similar internally to the old Vibe210 which was rated at 75x2. Same fuse size, similar physical dimensions etc. As a matter of fact -- the vibe 210, 214, 215, and 218 are all very similar boards with varying degrees of over-ratedness.
My point is the 210 was rated at 75x2 and it was over-rated as well. The 218 is no where near 100w per channel. Probably not even at 2 ohms.