Why does calling gem.bat from a cmd file exit after the first call?

I'm calling gem commands in a Windows .cmd file, but it exits after the first command. What gives?

gem sources --add http://gems.github.com 
gem install h开发者_如何学Pythonaml

The gem command is actually a batch file in your PATH. For example, C:\Ruby192\bin\gem.bat. Calling a batch file like this from a batch/cmd file automatically exits the entire session at the end of the called batch file. There is a special command that returns to the calling batch/cmd file. Use the CALL command before every call to another batch file.

CALL gem sources --add http://gems.github.com
CALL gem install haml

I solved it by calling the first gem command through cmd.exe, but I'm still keen to hear explanations of what is going on.

cmd.exe /c gem sources --add http://gems.github.com   
gem install haml