Certified GKH Gardening Expert
Is there a new apple tree in the area? Sometimes when the correct pollinator to your apples get planted, they can cross pollinate, and you will have seeds.
Sometimes, this can be a naturally occurring issue. I don't see it as much of a concern, but it may be possible that it just didn't get pollinated last year. If someone happened to plant a compatible tree within the flying distance of the tree, then it can get pollinated like this.