Pdf and watermark print

I have a pdf document created with a scanner and with a watermrk apllied.If I print this document with a normal print the text behind the watermark it's not readable.If I print this document with pdfcreator then with the printer the text now is readable.

The  watermark it's created both with adobe professianl or itextsharp

I think pdfcreator has some logic to fix the problem can you give me an idea how pdfcreator handle this?