iTerm2 applies strange color to vim's default
Because set termguicolors in .vimrc is not set. 

This is how I understand it; correct if I am wrong. 
A terminal emulator may either use ANSI colors or the terminal's own color palette. 
Mac's native Terminal can only achieve the former; iTerm2 can do both. 
With termguicolors off, iTerm2 uses ANSI colors. 
After termguicolors is set on, it uses its own GUI colors. 
Colors becomes more vivid, as is in the screenshot. 

Only when I converted to "solarized" color scheme did this suddenly occur to me. 
Still, you can see in the picture, it seems that newer version of iTerm2 has lighter color. 
Never mind. I am a "solarized" fan now.