In a capital case, a verdict to condemn must be by a majority of at least two. If the sanhedrin votes 12-11 to condemn they add a pair of judges and vote again, and keep doing this until they reach the full size of 71. This is all explained in chapter 5 of Sanhedrin.
We also learn in chapter 4 that, in a capital case, a judge who votes to convict can change his mind, but one who votes to acquit cannot -- once you acquit, your vote is locked in.
Against this backdrop we have the end of Sanhedrin 5:5:
If thirty-six acquit and thirty-five condemn, he is acquitted. But if thirty-six condemn and thirty-five acquit, the two sides debate the case together until one of those who condemn agrees with the view of those who are for acquittal.
Why do they continue debating at this point? The court cannot increase beyond 71, 35 votes are locked in, and the other 36 are not enough to convict. The case cannot end in conviction at this point. But instead of acquitting on the spot they continue debate until one of those who voted to convict changes his mind and the accused is acquitted.
The conclusion is foregone; why drag it out? Is there some extra benefit, spiritual or societal, to a "full" acquittal over this "not a big-enough majority" acquittal?