Am I looking at the wrong tables or have you quoted the wrong poll? Economist/YouGov's weekly is showing 54 Clinton 37 Sanders for me,
here.
EDIT: it's 61-31 among Democrats, Holmes, which isn't the same as likely Democratic voters.
That explains the gap, then.
EDIT: That's +1 for Clinton and -2 for Sanders since the last Economist/YouGOV poll. They're fortnightly and that gap was over the Christmas break, so I'm fairly sure that's just variation in the margin of error because nothing has really happened to do anything else. Going off both Fox and YouGOV, I think we can be fairly confident that 54-38 is probably an accurate representation of the state of the race right now.
EDIT: Sanders' favourables are +18 very favourable, +25 somewhat favourable, -16 somewhat unfavourable, -23 unfavourable, for net +4. Lower than he's usually had from these things, he's normally at around +8. Clinton's are +21 very favourable, +20 somewhat favourable, -12 somewhat unfavourable, -42 very unfavourable for net
-13. Again, lower than her normal -8. Probably a slightly more Republican sample than normal given it affected both of them, but still...
...she overtook Romney and Bob Dole for least liked presidential candidate of the postwar era! Only 1992 Bush to go. #Queen.