function.php

<?php
// \App\Model\Table\PollAnswersTable
public function countVotes ($id)
{
    /** @var \App\Model\Entity\Poll $poll */
    $poll = $this->get($id, ['contain' => 'PollAnswers']);

    // Some logic here, it's irrerevant

    // Calculate the score
    foreach ($poll->poll_answers as $answer) {
        $score = $poll->users_count === 0 ? 0 : $answer->votes_count / $poll->users_count;

        if ($answer->score !== $score) {
            $answer->score = $score; // <---------------------- This isn't saved
            $poll->dirty('answers', true);
        }
    }
    
    // Save the poll
    if (!$this->save($poll)) {
        throw new InternalErrorException(__('Failed to update the votes counter'));
    }
}

tables.php

<?php
class PollsTable extends Table
{

    /**
     * Initialize method
     *
     * @param array $config The configuration for the Table.
     * @return void
     */
    public function initialize (array $config)
    {
        $this->table('polls');
        $this->displayField('title');
        $this->primaryKey('id');

        $this->addBehavior('Timestamp');

        $this->hasMany('PollAnswers', [
            'foreignKey' => 'poll_id'
        ]);
    }
}


class PollAnswersTable extends Table
{

    /**
     * Initialize method
     *
     * @param array $config The configuration for the Table.
     * @return void
     */
    public function initialize (array $config)
    {
        $this->table('poll_answers');
        $this->displayField('title');
        $this->primaryKey('id');

        $this->addBehavior('Timestamp');

        $this->belongsTo('Polls', [
            'foreignKey' => 'poll_id',
            'joinType' => 'INNER'
        ]);
        $this->hasMany('PollVotes', [
            'foreignKey' => 'answer_id'
        ]);
    }
}