There are always early and late babies. I saw plenty of young in the early spring and had a LOT of squirrels at my place this year. Raccoons have also been prolific. You may be seeing a second batch on the squirrels or you may be seeing squirrels born early having their first litter.
Keep in mind that there are a lot of things that will influence births including food, rain, predation pressure, temperatures, etc. Also keep in mind that there are always going to be some early and late births. That is part of nature's was of assuring that at least some of the generation will survive until next year.