The link I provided above covers this and suggests that the foundry that created Arial didn't want to be a blatant pirate, so instead did just enough to pretend they weren't ripping off Helvetica.
"What is really strange about Arial is that it appears that Monotype was uncomfortable about doing a direct copy of Helvetica. They could very easily have done that and gotten away with it. Many type manufacturers in the past have done knock-offs of Helvetica that were indistinguishable or nearly so. [...] It’s quite possible that most of the “Helvetica” seen in the ’70s was actually not Helvetica.
Now, Monotype was a respected type foundry with a glorious past and perhaps the idea of being associated with these “pirates” was unacceptable. So, instead, they found a loophole and devised an “original” design that just happens to share exactly the same proportions and weight as another typeface.
This, to my mind, is almost worse than an outright copy. A copy, it could be said, pays homage (if not license fees) to the original by its very existence. Arial, on the other hand, pretends to be different. It says, in effect “I’m not Helvetica. I don’t even look like Helvetica!”, but gladly steps into the same shoes. In fact, it has no other role."
