Polling is very good for many things. Comparing hypothetical candidate matchups to actual candidate matchups is not one of them, for the straightforward reason that when someone turns from a hypothetical candidate to an actual candidate, attacks start and their favorability drops.
Case in point: the long timeline of Hillary Clinton's approval polling. Favorability dropped when she became a candidate for senate, rebounded when she was SecState and not a candidate, dropped when she ran for President. news.gallup.com/poll/224330/...
