In fact, since last Sunday's CNN/ORC poll, the change Trump has seen since the last polls from each outlet has varied — up since the last one in some, down in others.
We've noted before that reviewing polling is like trying to figure out who's winning a horse race based on photographs you're given every 100 yards. To the pedants' point, a photo of the pack just out of the gates doesn't tell you much about who's going to win — but if a long shot comes out of the gates strong and, in a photograph taken 200 yards later, has widened his lead, that's interesting.
So what we have here is a number of photos taken at various times that we're comparing to one another. NBC and the Wall Street Journal took one in late July that showed Trump at 19 percent. They took another one more recently (the poll was completed Sept. 24) that showed him at 21. What we don't see is what happened in the meantime.
Happily, those other polling firms were taking snapshots of the race, too. And if we put them in the proper order, it looks like this.
That gives us a better picture. The folks at Real Clear Politics create a running average of the polls, a sort of mock horse race based on the photos we've seen. (They exclude some of the blurry photos from polling firms that use unproven methodologies or have a bad track records.) If we include the RCP polling average, you can see more clearly what happened.
According to the glimpses of the race we've gotten, Trump was at 19 in July and then surged ahead. He led big. The Washington Post-ABC News poll that was completed Sept. 10 had Trump at 33 points! But recently he's faded back into the pack. Now he's at 21 percent, when NBC took another photo. Doing better than he was in their last photo — but not overall.
To beat a ... well, to exhaust the analogy, all of these photos are somewhat blurry. That is, the margins of error mean that there are a lot of ways in which candidates might not be exactly where a poll suggests. But again, that's why it's useful to consider the grand sweep of the information at hand. And with all of that information, Trump being up two points since a poll in July is not something that's actually good news for his campaign.