http://www.pressgazette.co.uk/nrs-t...nd-online-with-mail-still-top-on-29m-a-month/ has the Guardian at number 4 most read newspaper, but only about 10-15% behind second and third. Given that the vast majority of readers are online, and the three ahead of it are relentlessly clickbait-based a huge proportion of their views are likely to be low-engagement, and people will tend to regard them as "reading facebook" or wherever they clicked on the link rather than "reading the mail".
There's no reason at all why a poll of literate internet users interested in trains should precisely match national averages, but it's probably only different from the general population by a factor of 2, rather than being as insane as you suggest.
Incidentally, the losses for last year were around £190k/day. This year's figures are out in a month or so and will be down substantially (albeit higher with redundancy costs). The Sun lost a similar amount in the same year, and the Times also lost money. Rubbish business, newspapers