rake deploy with permission denied when building Github blog

Question

I build Github blog based on Octopress and I have already added SSH key as Github's instruction. And when I test ssh -T git@github.com in git shell, it return successful result with

Hi xxx, You're successfully authenicated ...

while when I run rake deploy in Ruby command, it will return Permission denied (PublicKey)

I have searched in Google and answers are always about wrong setting in SSH, but I don't know why ssh -T operation is successful in git shell if I set its wrong.

Please kindly give me some ways if you have ever met similar problem, thanks.