As we found earlier in the case of pageviews (cf. T148663#2764417 ), the iOS app user agent changed from containing iPhone to containing iOS back in September 2016. The pageviews code was updated, but not the Oozie jobs calculating daily and monthly uniques.
The issue is also affecting the Android numbers, because the queries assume that everything that is not iOS is Android:
CASE WHEN user_agent LIKE('%iPhone%') THEN 'iOS' ELSE 'Android' END AS platform
(which BTW seems somewhat questionable in itself, but that's for another day).
It looks like this bug is currently inflating Android daily uniques by up to 10% or more.