为什么 APT 不从我的本地镜像下载最新版本的软件包？

Question

我已经在我的局域网上设置了一个 Ubuntu 14.04 apt-mirror，使用本指南一切似乎都运行正常，只是今天我注意到在我的 repo 中，我有一个较新的 bash 版本可用，但 apt 似乎无法在我的客户端上看到它（请注意...ubuntu1.6_amd64.deb。当我从我的 apt repo 下载 bash 时，客户端会下载...ubuntu1.5_amd64.deb而不是 1.6。为什么？

# dir ubuntu/pool/main/b/bash/bash_4.3-*
-rw-r--r-- 1 root root 574464 Apr  8  2014 ubuntu/pool/main/b/bash/bash_4.3-6ubuntu1_amd64.deb
-rw-r--r-- 1 root root 575658 Oct  9  2014 ubuntu/pool/main/b/bash/bash_4.3-7ubuntu1.5_amd64.deb
-rw-r--r-- 1 root root 575388 Nov 17 17:53 ubuntu/pool/main/b/bash/bash_4.3-7ubuntu1.6_amd64.deb

在我的 LAN 客户端上：

    # aptitude update
    ...

    # aptitude show bash | grep Version
    Version: 4.3-7ubuntu1.5

    # aptitude download bash
    Get: 1 http://MYLANHOST/ubuntu/ trusty-updates/main bash amd64 4.3-7ubuntu1.5 [576 kB]
    Fetched 576 kB in 0s (25.3 MB/s)

更新：显示apt-cache policy bash

# apt-cache policy bash
bash:
  Installed: 4.3-7ubuntu1.5
  Candidate: 4.3-7ubuntu1.5
  Version table:
 *** 4.3-7ubuntu1.5 0
        500 http://MYLANHOST/ubuntu/ trusty-updates/main amd64 Packages
        500 http://MYLANHOST/ubuntu/ trusty-security/main amd64 Packages
        100 /var/lib/dpkg/status
     4.3-6ubuntu1 0
        500 http://MYLANHOST/ubuntu/ trusty/main amd64 Packages

更新：显示时间戳Packages.*

# dir ubuntu/dists/trusty-updates/main/binary-amd64/                    
total 8224
drwxr-xr-x 2 root root    4096 Oct 11 05:55 .
drwxr-xr-x 4 root root    4096 Oct 11 05:55 ..
-rw-r--r-- 1 root root 6341067 Nov 21 09:30 Packages
-rw-r--r-- 1 root root     104 Nov 21  2016 Release
-rw-r--r-- 1 root root 1144628 Nov 21  2016 Packages.gz
-rw-r--r-- 1 root root  917444 Nov 21  2016 Packages.bz2

# cat ubuntu/dists/trusty-updates/main/binary-amd64/Packages | grep bash
Package: bash
Replaces: bash-completion (<< 20060301-0), bash-doc (<= 2.05-1)
Recommends: bash-completion (>= 20060301-0)
Suggests: bash-doc
Conflicts: bash-completion (<< 20060301-0)
Filename: pool/main/b/bash/bash_4.3-7ubuntu1.5_amd64.deb
...