> It's a bit of a double standard, but they might have been expecting you to be dressed in a suit anyway.

I've encountered this. My current workplace has a notoriously laid back and straight-forward culture. Knowing this, I asked the interviewer who was inviting me to interview if I should dress up, or dress appropriately for the company culture. "I would recommend you dress up a little, and hope it's the last time you have to wear those clothes," was the response.

Showing that you are familiar with the culture into which you're walking is worth a lot in such situations.