#883080 Wrong version "already installed" when -t is used

Package:
aptitude
Source:
aptitude
Description:
terminal-based package manager
Submitter:
積丹尼 Dan Jacobson
Date:
2017-11-29 11:45:04 UTC
Severity:
normal
#883080#5
Date:
2017-11-29 11:30:18 UTC
From:
To:
# set firefox
# apt-cache policy $@
firefox:
  Installed: 58.0~b4-1
  Candidate: 58.0~b4-1
  Version table:
 *** 58.0~b4-1 990
        990 http://free.nchc.org.tw/debian experimental/main amd64 Packages
        100 /var/lib/dpkg/status
     57.0-1 500
        500 http://free.nchc.org.tw/debian unstable/main amd64 Packages
# aptitude -t unstable install $@
firefox is already installed at the requested version (58.0~b4-1) ## W R O N G ##
# aptitude -t experimental install $@
firefox is already installed at the requested version (58.0~b4-1)
# dpkg -r firefox
Removing firefox (58.0~b4-1) ...
# aptitude -t unstable install $@
The following NEW packages will be installed:
  firefox
Setting up firefox (57.0-1) ... ## finally correct ##

APT::Default-Release "experimental";//just order them in sources.list UNTRUE
APT::AutoRemove::RecommendsImportant false;
APT::AutoRemove::SuggestsImportant false;
APT::Cache::AllVersions false;
APT::Clean-Installed false;
APT::Get::Fix-Missing true; //even though 825897 says should be off
APT::Install-Recommends false;
Aptitude::CmdLine::Always-Prompt true;