[dubcanada]

Yes, as someone said below, what's the difference between most programming languages? Very small changes in the grand scheme of things, they mostly all have variables, classes, functions, etc. It's the same thing with fonts, they mostly all have the same sort of form, with variations. Put that font beside Arial and Helvetica and you can probably see similarities between them as well, yet they have a different name.

[Samuel_Michon]

Arial should have never come to be. The only reason it exists is because changing it a little and giving it a new name meant Monotype didn’t have to pay Linotype license fees.

In this case, Spiekermann made both fonts.

[ZeroGravitas]

Arial was originally it's own font (Monotype Grotesque), but got squished to match Helvetica's metrics which is a good example of how fonts can be different and similar to each other.

[Samuel_Michon]

That is true, here’s a fun comparison: http://www.marksimonson.com/notebook/view/how-to-spot-arial

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.