Incorrect rendering when /Matte entry

The image softmask in the attached file has a /Matte 0 0 0 entry. PDFBox displays the PDF differently than Adobe Reader, the reflection shown by PDFBox is barely visible. When the /Matte entry is deleted, then it is barely visible in Adobe Reader too. So the /Matte entry does make some difference, although I don't understand how.

In the PDF specification, the formula shown is c' = m + α x (c - m). So 0 should have no effect?!

I looked at the code of PDF.js, they have a special handling when alpha is 0, don't know why.