Yes, it's because tabs vary depending on configuration that projects, such as this, have set the code style to be spaces instead of tabs. That way everything looks the same regardless. As you see GitHub display tabs as 8 spaces - which mess up the diffs. It would also mess up the 80 character line width rule.

(Btw, I take it you reply by email? The reply is posted back to GitHub and the old replies are visible. Can you erase the old content of the emails when replying? Makes it easier to follow the comments in the pull request.)