[abdias]

Deflating can be applied to some chunks, but not at will. The zTXt chunk can be compressed while for example the tEXt chunk cannot. The newer iTXt chunk can vary.

The two former are limited in scope and language encoding support, so iTXt is typically used for extended textual data such as XML/XMP etc. But if is saved compressed or not depends on the PNG encoder/host used (there can also be multiple instances of these chunks in the same file).

Photoshop for instance saves uncompressed, I guess to give fast access for performance reasons (ie. file viewers using galleries for numerous images while displaying their meta-data).