They are "very much similar" only in that they have compatible metrics. Thus, text typeset in Times, Arial, and Courier won't have to re-flow when using Tino, Arimo, and Cousine. You're absolutely right; the style of the fonts is quite different than the style of the one's they are intended to replace.

From the font info for Arimo:

    Unique name	Ascender - Arimo
    Copyright	Digitized data copyright (c) 2010 Google Corporation. 
    Trademark	Arimo is a trademark of Google and may be registered in certain jurisdictions.
    Description	Arimo was designed by Steve Matteson as an innovative, refreshing sans serif design that is metrically compatible with Arial™. Arimo offers improved on-screen readability characteristics and the pan-European WGL character set and solves the needs of developers looking for width-compatible fonts to address document portability across platforms.
    License	Licensed under the SIL Open Font License, Version 1.1