There are usually dropouts, and bigger sample is better than smaller. In larger advanced trials (2B and 3) biotechs will establish a floor (ie minimum) sample target that meets what they and the FDA (and possible BP partners) would deem "adequate", with room for additional patients as contingency.
Why would they recruit 10 addtl patients? Anybody?
A couple of reasons: 1) possibly replace dropouts and maintain a number considered reliable to BP and others(as already mentioned); 2) increase the sample size for either arm, 300mg or 400mg; 3) however, most likely but needs confirmation, add reliability to the 400mg arm if it showed greater efficacy than 300mg. I say it's more likely because there were 27 in the 400mg group and increasing the N to 35 or more would further confirm anything they saw against the placebo. IMO, there would be no reason to add more test subjects to the 300mg arm since the N=81 would already be sufficiently powered ahead of a pivotal trial. Also, 10 more strengthens the safety data and better insures approval of a Phase 3 at the higher dose.
Recall that the previous trial results showed dose dependence. 10 more would further confirm any perceived increased efficacy.