Pierce Crystal Oscillator

Stupid me! I should have known.... After some RTFL (literature) the mystery about my 74HC86 oscillator seems to be solved. The high frequency is not due to the inner values of the XOR-gates, it is actually caused by the chosen oscillator type "Pierce". In the literature it reads that a Pierce crystal oscillator does oscillate above the crystals series resonant frequency. There we go... I should/could have known that one....