Easy, passmark is still optimized only for Intel, cinebench isn't.
You'll run into this in other programs too. Some the 1700x will beat the ,6900 some will lose handily. As windows fixes it's scheduled to favor full cores over logical ones the number will go down, and as programs optimize for the 1700x I don't think many programs will favor the 6900k in multithreaded loads. If you have absolutely no budget get a 6900k if not the additional GPU/memory/ssds you can buy with the savings will make the 1700x system stronger overall