Thanked after PhD interview although it went really badly

The PhD supervisor emailed to thank for interviewing the evening after the afternoon interview.

I know it went really badly so I was surprised he bothered.

Do they tend to thank all applicants?

(I think I look like one of the strongest applicants on paper, but I suck at interviews, so perhaps this is why he bothered?)

Did you reply back? You could have made a renewed pitch returning the compliment mentioning the imperfect interview and your continued interest.

I replied back just saying thank you.
I didn''t say anything about an imperfect interview, this seems a bit unprofessional, everyone can try to claim their interview wasn't their best etc, I think leave the board to decide whether nerves got the better of someone they see potential in or not, do you think I should have?

Its done now so move on.

Personal choice, but it is a small chance to sell yourself again and correct any glaring errors. It is more usual for the interviewee to send the thank you, so take it as mildly positive and move onto the next opportunity. It will take care of itself.

k thanks.
the interview ended up being skype rather than in person due to the snow that week, so it ended up as a skype interview that afternoon, i guess there were some things that, they didn't understand what I meant when deriving something, which, had I been there in person I could have wrote down and I'm sure it would have made more sense then, but all too late to mention now, no point sending another thank you and look forward to hearing from you email again?