POSTGRESQL insert after trigger causing id of inserted record to not increment sequentially

Question

I'm working on an insert after trigger in Ruby on Rails using the hairtrigger gem, and the trigger is successfully inserting a record into the DB but for some reason the value of the previously inserted record's messages_count column is being added to the previous record's primary key value to calculate the id for the new record instead of a normal incrementation of 1.

Here's the hairtrigger definition:

trigger.after(:insert) do
<<~SQL
  INSERT INTO campaign_stats (campaign_id, messages_count)
  SELECT
      campaign_id as campaign_id,
      1 as messages_count
      FROM campaigns
      INNER JOIN campaign_subscriptions
      ON campaigns.id = campaign_subscriptions.campaign_id
      INNER JOIN conversations
      ON campaign_subscriptions.id = conversations.campaign_subscription_id
      WHERE conversations.id = NEW.conversation_id
      LIMIT 1
  ON CONFLICT (campaign_id) DO UPDATE
  SET
    messages_count = campaign_stats.messages_count + EXCLUDED.messages_count;
SQL


end

And some evidence of the incrementation that's happening using the messages_count:

    CampaignStat Load (0.3ms)  SELECT "campaign_stats".* FROM "campaign_stats"
=> [#<CampaignStat:0x00007fd2da9d96a8 id: 1, campaign_id: 1, messages_count: 5>,
 #<CampaignStat:0x00007fd2da9d9540 id: 6, campaign_id: 2, messages_count: 10>,
 #<CampaignStat:0x00007fd2da9d9400 id: 16, campaign_id: 3, messages_count: 5>,
 #<CampaignStat:0x00007fd2da9d9298 id: 21, campaign_id: 4, messages_count: 2>]

I'm unsure of what portion of my SQL could be causing this issue. Any help would be much appreciated.