[ECI-EN] Questions about PDF/X standards

Dear Sir or Madam, my name is Tjaša and I am studying at the university of Natural Sciences and Engeneering in Ljubljana (Slovenia). I am allready writing a degree in Graphic technology about PDF/X standards and differences between them. I made my own test form to compare different PDF/X standards and different print devices. I have exported differend PDF/X standards from Adobe Indesign CS3 and then I have printed them on different printers (digital printers). Although I was looking for many answers on the internet I didn't find out why there are differences between standards in LAB (differences between PDF/X-3 and PDF/X-4, as I traved an information which mentioned that PDF/X-3 and PDF/X-4 interpret LAB colors in the same way). I also find out that PDF/X-1a and PDF/X-3 are smilar and PDF/X-4 and other presets in Adobe Indesign (such as press quality and high qualilty print) are similar too in interpreting the LAB colors. I just didn't find out why. The same problem happened with overpint. Only PDF/X-4 and high quality preset were printed suitable. The PDF/X-3 and PDF/X-1a didn't recognize the overprint. Similar happened with pictures (one in CMYK and the other in RGB color space).. PDF/X-3 made them really similar while PDF/X-4 made RGB printed pictures much more bright instead they have the same setting about converting colors during export. Is it possible to explain this with differences between standards or it depends on the settings of the RIP? I will really apreciate any answer you could give me. It wiil be very helpful. Best regards Tjaša Uršič