Piwik currently has an issue with correctly detecting an iPhone 5 and shows a wrong display size in the logs:

iPhone / Safari / 320x568

The correct size would be 1136 x 640px.

Find the iPhone5 user agent string below:

Mozilla/5.0 (iPhone; CPU iPhone OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A405 Safari/8536.25