UICollectionViewのセルが左寄せになるときと中央寄せになるときがある

質問

どうして左寄せになる場合と、中央寄せになる場合があるのでしょうか？
下記にそうなってしまったコードを載せます。

左寄せ

import UIKit
class ViewController: UIViewController {
    @IBOutlet weak var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.collectionView.delegate = self
        self.collectionView.dataSource = self
        let layout = UICollectionViewFlowLayout()
        layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        layout.minimumInteritemSpacing = 0
        layout.minimumLineSpacing = 0
        self.collectionView.collectionViewLayout = layout
    }
}

extension ViewController: UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 3
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = self.collectionView.dequeueReusableCell(withReuseIdentifier: "hogeId", for: indexPath)
        cell.backgroundColor = indexPath.row % 2 == 0 ? UIColor.green : UIColor.blue
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

        return CGSize(width: self.collectionView.frame.width / 2, height: 100.0)
    }
}

中央寄せ

import UIKit

class ViewController: UIViewController {
    @IBOutlet weak var collectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()

        self.collectionView.delegate = self
        self.collectionView.dataSource = self
        let layout = UICollectionViewFlowLayout()
        layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        layout.minimumInteritemSpacing = 0
        layout.minimumLineSpacing = 0
        self.collectionView.collectionViewLayout = layout
    }
}

extension ViewController: UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 2
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = self.collectionView.dequeueReusableCell(withReuseIdentifier: "hogeId", for: indexPath)
        cell.backgroundColor = indexPath.row % 2 == 0 ? UIColor.green : UIColor.blue
        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

        if indexPath.row == 0 {
            return CGSize(width: self.collectionView.frame.width, height: 100.0)
        } else {
            return CGSize(width: self.collectionView.frame.width / 2, height: 100.0)
        }
    }
}

シミュレータの実行結果画像