Ludvig,
It's hard to say what is different without seeing a sample file.
PM looks at a number of different bits of metadata for time. The Exif tag 0x9003 written by the camera is typically considered the true value, but there are other Exif/TIFF tags that can be considered as well as XMP (e.g. Photoshop's DateCreated).
Can you provide a sample file?
--dennis