Hi megibyan,
XMP has categorization of exif properties. Some properties if not present can be only read but can't be deleted or modified. But if that is not present, you can add it. example of those properties are ApertureValue, BrightnessValue, ExposureBiasValue etc. As your file have already ApertureValue property present, you are not able to modify it.
To know more about these properties and categorization of the property please have a look on toolkit\XMPFiles\source\FormatSupport\ReconcileTIFF.cpp\ReconcileTIFF.cpp in SDK.
If you want to modify, delete it first (using some other utility e.g. exiftool) and add the same property with new value.
-Sunil