Those look pretty different to me. I mean, very clearly related, but with tons of small changes. Like, the angle of almost every terminal is different, and in general the new typeface is much more vertical.
In this case, Spiekermann made both fonts.
As you can see, all the areas in which Monotype Arial is different from Linotype Helvetica are borrowed from Monotype Grot. The reason to make such a monster is purely financial.