Because the bias was diluted by the other jurors - from the passage on page iii, I assume the juries in the study were only 10-33% BME. The study follows your quote with:
Even though the defendant’s ethnicity did not have an impact on jury verdicts, the research found that in certain cases ethnicity did have a significant impact on the individual votes of some jurors who sat on these juries. Statistical analysis of the individual votes of all 319 jurors who took part in the case simulation showed that in certain cases BME jurors were significantly less likely to vote to convict a BME defendant than a White defendant. [..]
The report concludes that this highlights the benefits of permitting majority verdicts and of having 12 member juries. The fact that 12 jurors must jointly try to reach a decision and that majority verdicts are possible meant that more verdicts were achieved and individual biases did not dictate the decision-making of these racially mixed juries. If juries were smaller or if unanimous verdicts were required, then individual juror bias might potentially have a greater impact on jury verdicts.
Even though the defendant’s ethnicity did not have an impact on jury verdicts, the research found that in certain cases ethnicity did have a significant impact on the individual votes of some jurors who sat on these juries. Statistical analysis of the individual votes of all 319 jurors who took part in the case simulation showed that in certain cases BME jurors were significantly less likely to vote to convict a BME defendant than a White defendant. [..]
The report concludes that this highlights the benefits of permitting majority verdicts and of having 12 member juries. The fact that 12 jurors must jointly try to reach a decision and that majority verdicts are possible meant that more verdicts were achieved and individual biases did not dictate the decision-making of these racially mixed juries. If juries were smaller or if unanimous verdicts were required, then individual juror bias might potentially have a greater impact on jury verdicts.