Thank you for your growing interest in Aspose.Slides.
I have been able to observe the same value to both, the ppt and pps as zero (see attached image). I am discussing this with our product team and will be able to share our findings soon. We appreciate your patience in this regard.
I have discussed the issue with our product team and like to share with you that, Ppt and Pps have same value. But this is not a bug. The reason is that it is really the same file, except file extension (.ppt and .pps). You can simply rename the extension in file browser and PowerPoint 2003 (ppt) will be PowerPoint 2003 Show (pps), and vice versa.
I hope this will clarify the concept. Please share if I may help you further in this regard.