As part of a minority group (I don't mind the term applied to myself as there aren't even enough individuals globally who share my ethnicity to fill Emirates Stadium!), I'll add my two cents.
First, remember that one assessment centre group will not be representative of the firm's recruitment process. Unless you have further information, I don't think it best practice to assume that your experience is representative of the other ACs that the firm might run over an application cycle - chance plays a large role in this; also, the smaller the AC group and the more ACs run per cycle, the greater the chance that the group might be more homogenous than one might intuitively expect or hope for.
Second, regarding the 'lack of guys'. Well, if we skip past my explanation in the proceeding paragraph, another explanation is simply that there just so happened to be more female candidates who passed the prior stage of the application process. I think candidates are quick to jump the gun (on either side of the debate) and claim that law firms are racially or positively discriminating based on the gender/ethnicity split of one AC group. I do not wish to comment on whether such discrimination does or does not occur; however, I think it is worth highlighting the intuitive, but often glossed over point, that a central reason, admittedly amongst others, for a candidate getting to the AC stage, is their exceptional ability.
Third, regarding the lack of BAME candidates in ACs. Again, I think my first and second points give some explanation; however, I will say that, in the past, I have had experiences related to commercial law that made me feel, as a person from a minority ethnic background, like I stuck out. Nevertheless, this was not in the context of ACs/VSs - every time I have interviewed at a firm, there has always been a good mix of female, male, BAME and white candidates. What I have noticed, however, is a lack of candidates who are both female and BAME. I am not such an individual, so I cannot make any more of a comment on that, other than that has been my historical observation. I think a greater awareness and sensitivity towards intersectionality would be a most welcome addition to any workplace.
Perhaps a more worrying point, which I am glad Allen & Overy has led the way in addressing, is the 'stay gap' between BAME and white lawyers. This is the statistic that lawyers who identify as BAME often leave their firms earlier than their white counterparts. Such statistics should act as an impetus for change and a more inclusive workplace, and inform better recruitment and training practices. A lot of the dialectic is often constrained to the recruitment level; whilst this is most certainly warranted, I think more emphasis should be placed on the entire ladder, from vacation schemer to partner, if you will.
Anyway, that is a disgustingly verbose 'two cents' - I do apologise!