Difference in output bit of AD9643-250 according to sample rate

I am using AD9643-250 as ADC. 
The same signal (ex. 1 MHz sine wave of 0.5 V) entered into the ADC, but the output bit of the ADC depends on the sampling rate (240 or 40 MHz).
When using 240 MHz sampling rate, the bit value of Vpeak is smaller than when using 40 MHz.
The RC network at the front of the ADC input is used by changing C1 to 10 pF by recommending using R1=33, C1=8.2 pF for the 0 to 100 MHz input signal.
Can the output bit value vary depending on the sampling rate?