Gain of ADL5330 for low frequency


My customer wants to use frequency of ADL5330 from 20MHz to 1GHz.

She measured S21 of ADL5330 evaluation board by network analyzer as below(Vgain=1.4V).

In the above measurement result, ADL5330 gain at 20MHz is 8dB.

But, ADL5330 gain at 20MHz on datasheet is about 21dB as below.

How much is ADL5330 gain at 20MHz?

If the datasheet is correct, how do I improve the gain for low frequency in ADL5330 evaluation board?

Thanks a lot.