I feel like we've had this discussion so many times, and there's something about it that just doesn't click for some German speakers. related discussion: to be wide of the mark - völlig daneben liegenrelated discussion: wide of the mark - weitab vom Zielrelated discussion: dab her eyes / They're all wide of the markrelated discussion: wide of the truth
Maybe it's that you're mistakenly assuming that 'ab(seits)' should mean 'off,' when really that's the meaning of 'wide'?
That is, 'wide' here doesn't mean 'breit' or 'weit,' it means to one side, in a sideways direction, to the left or right, horizontally (as opposed to up or down, high or low, vertically).
'Of' relates that direction to an intended point; here, the target, the mark. The meaning is similar to 'from,' but here, 'of' is idiomatic and 'from' isn't. I wonder if in German you could even say 'von X ab'? Sorry, it's easier for me to describe than translate.
You can also miss a target in other directions -- you could be high of center, high of where you were aiming, short of the desired amount, short of the goal line, short of the base in baseball, short of the hole in golf, etc.
You can also be wide left or wide right, to specify which side, though those don't usually combine with 'of' except separately -- wide of center, right of center, but just wide right.
'Low of' is probably less common, but a musician could conceivably be low of the center of the pitch, or a shot could be low of center, even though under or below would be more typical, just as over or above would be more frequent than 'high of.' Similarly, 'long of' is also not common, but probably thinkable -- in American football, a pass could conceivably be long of the intended receiver, though past him would be more common.
I'm not sure '*widely off' really helps, since that's not idiomatic -- we would just say far off, a long way off, well off base, etc.
But we're not using 'off' here, so maybe the best thing is just to try not to think about it. (-: