I'm interpreting this as you rec'd data sets from two separate sources, so they are not the same. You "made" them the same and combined.
Now when you sort, they are still "top-half" and "bottom-half" separated ...?

Check to see that your zips are all the same data type - either numeric or text (if it is the xxxxx-xxxx format)
Especially if it is text, check for leading spaces.