"Why bother with brand consistency? Brand consistency has one massive advantage – recognition. With recognition comes familiarity. With familiarity comes trust and confidence." (http://www.attitudedesign.co.uk/the-no-1-rule-brand-consiste...)

Each app can (and probably does) develop along its own path, but always stays somewhat in line with their "continuous experience." When you start questioning if it's a Google product or not, they're probably doing something wrong. The Google Experience has become part of their value prop - why give that up?