Tmux always modify the zsh PATH

I'd like to use tmux inside my iTerm2.

and I have the following in my ~/.tmux.conf file.

set-option -g default-shell /bin/zsh

set-option -g default-command "/bin/zsh -c 'which reattach-to-user-namespace >/dev/null && exec reattach-to-user-namespace /bin/zsh -l || exec /bin/zsh -l'"

In my iTerm2, I set /bin/zsh to my default shell too.
The output of echo $PATH inside iTerm2 but outside tmux is the following:

/Users/alex/.pyenv/plugins/pyenv-virtualenv/shims
/Users/alex/.pyenv/shims
/Users/alex/tools/gradle-2.13/bin
/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/bin
/Users/alex/.pyenv/bin
/usr/local/bin
/usr/bin
/bin
/usr/sbin
/sbin

but inside tmux, echo $PATH prints the following:

/usr/local/bin
/usr/bin
/bin
/usr/sbin
/sbin
/opt/X11/bin
/Users/alex/.pyenv/plugins/pyenv-virtualenv/shims
/Users/alex/.pyenv/shims
/Users/alex/tools/gradle-2.13/bin
/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home/bin
/Users/alex/.pyenv/bin

It seems that tmux has put the /usr/local/bin etc in front of my PATH setting, which makes pyenv can not find the correct python.

My ~/.zshenv is :

alias vim="nvim"
PATH="/usr/local/bin:$PATH"

export PYENV_ROOT="$HOME/.pyenv"
PATH="$PYENV_ROOT/bin:$PATH"

export JAVA_HOME=/Library/Java/JavaVirtualMachines/jdk1.8.0_45.jdk/Contents/Home
PATH=$JAVA_HOME/bin:$PATH
export CLASSPATH=.:$JAVA_HOME/lib/dt.jar:$JAVA_HOME/lib/tools.jar

export GRADLE_HOME=/Users/alex/tools/gradle-2.13
export PATH=$GRADLE_HOME/bin:$PATH


export HOMEBREW_GITHUB_API_TOKEN=xxxxxxxxxxxxxxxxxxxxxxxxxxx

eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"

and my ~/.zhrc is :

export ZSH=$HOME/.oh-my-zsh
ZSH_THEME="robbyrussell"
plugins=(git)

source $ZSH/oh-my-zsh.sh

export LANG=en_US.UTF-8

if [[ -n $SSH_CONNECTION ]]; then
  export EDITOR='vim'
else
  export EDITOR='nvim'
fi

So what is wrong here?

Do I miss something during tmux starts up?