To be precise, calibre compares the ModDate from the PDF Info dictionary to the MetadataData in the XMP block. In your problem PDF, the ModDate is Mon Apr 6 23:24:42 2015 and the MetadataDate is 2014-04-22T00:53:01+02:00
so calibre will use the information from the Info block rather than the XMP, since the Info block is marked as being newer.
To be precise, calibre compares the ModDate from the PDF Info dictionary to the MetadataData in the XMP block. In your problem PDF, the ModDate is Mon Apr 6 23:24:42 2015 and the MetadataDate is 2014-04- 22T00:53: 01+02:00
so calibre will use the information from the Info block rather than the XMP, since the Info block is marked as being newer.