I am sure there are per-user resource limits. In fact I know there are because the host documents then. But I'd assumed they would cover all my sites within the same account. I have a hosting account with add-on domains. Both the production and test sites are add-on domains and there are others. One of the others gets much more traffic than the one that is generating errors - sometimes up to 50 times more traffic when we have some ad campaigns running and they don't give an error.
I've checked the bandwidth logs and we never go above 15% of the allowance.