Updated TUs not merging

Two TUs appear for one translation. One edited slightly. Both are from the same local file-based TM. When I went to batch tasks update main TM, had the TM containing the two units set to "update" and had the batch settings at merge, it still gave me this in the translation result window. I could simply delete the unnecessary TU, but I was wondering why a new one was created in the first place. When I confirm segments, I use the simple ctrl+enter function, not the add new translation function.