To anyone interested,
I submitted a ticket for this, and have received an answer.
On 2.4GHz, the transmit power of the B One is 20dBm, vs 29dBm for the Surf Soho MK3. So that explains what I’m seeing.
On 5GHz, their respective transmit powers are 24dBm vs 23dBm, so the B One might have a slight edge on that frequency.
These figures are available on their website, although I hadn’t been able to locate the B One data as it’s not on their datasheet.