The frequency of the crystal oscillator connected to A111 is specified between 20-80MHz. It must be at least 20.625MHz in order for the SPI frequency to reach 50MHz which is necessary in order to obtain the 1500Hz update rate.
The main purpose of having a range for the input frequency on XIN between 20MHz and 80 MHz is to allow for a flexible hardware integration. It allows the customer to choose a crystal oscillator more freely and an already existing reference clock in the design could be reused for the A111. It must however be ensured that the crystal or the reference clock that is to be used fulfils the requirements stated in the datasheet.
The use of a crystal oscillator or reference clock with higher frequency could benefit the accuracy of the sensor measurement if the relative increase in phase noise is smaller or equal to the relative increase in frequency. The use of a crystal oscillator or reference clock with higher frequency will however increase the power consumption of the A111.