AD8351

It is a question about AD8351.
Using the evaluation board
Although the gain is set to 18 dB (RG = 50 Ω) as a single end, the gain characteristic flattens only to about 120 MHz in the frequency response. In the data sheet, it is a gain flat up to 400 MHz, but please let me know if there are things that can be considered about the cause. Thanking you in advance.