Bryan Bishop at The Verge:
As outlined on the Android Developers site, Google now uses the data collected when users visit the Google Play Store; under the previous system, any check-in to the store by the device would have been incorporated into the results, user-generated or not. The new system went into effect starting with this month’s results.
The change essentially skews the results towards those users who are actively visiting the Play Store. Google says as much on the page itself, noting that the new system “more accurately reflects those users who are most engaged in the Android and Google Play ecosystem.”