It seems that when you try to tap with one finger, the Two Finger emulation scrolling is invoked. This could be due to the default values of EmulateTwoFingerMinW, EmulateTwoFingerMinZ is not workable for you. You can try to change these values by running this in terminal, for confirmation
philipp
It seems that when you try to tap with one finger, the Two Finger emulation scrolling is invoked. This could be due to the default values of EmulateTwoFinge rMinW, EmulateTwoFinge rMinZ is not workable for you. You can try to change these values by running this in terminal, for confirmation
synclient EmulateTwoFinge rMinW=20 EmulateTwoFinge rMinZ=400