WordPress update changed term IDs

I recently updated my WordPress site from version 3.9.1 to version 4.5.1. Before the update, I had three terms within a taxonomy that had the IDs, of 4, 5 and 6. After the update, those IDs changed to 10, 11, and 12. A few things on my site depended on those ID numbers, so they broke when the IDs changed.

I reverted back to 3.9.1 several times and tried updating WordPress, along with updating multiple plugins, in different orders, but the only thing that changed these IDs was updating WordPress. I also noticed the change only happened on my terms. The IDs of my pages, posts and media attachments all stayed the same.

Why would a core WordPress update change the term IDs?