I think the difference is between what's stored in the database for the template title and what's actually sent via email.
So for instance:
{WEB_TITLE} - Edit options for the listing {ITEM_TITLE} is within the 255 char limit, but when resolved title is put in the email title before sending, it's cut off after 70 char because it exceeds the 70 char limit, which is probably hardcoded somewhere.