[delta_p_delta_x]

> the rhytm of the typeface

That's what the parent commenter means by 'They just matched the outer dimensions of Whitney'.

In other words, metrically compatible. To the untrained eye, metrically compatible typefaces all look the same, because they're meant to be swapped between each other. In my view Whitney and Source Sans couldn't be more different. The stroke cuts in Whitney are angled, they're perpendicular in Source Sans. The lowercase 'b', 'e', and 'g' are very different in both fonts.