When the merge took place, your other account has profiles on the following sites too:
- Super User
- Web Applications
- Information Security
Merging the network account together causes a merge to occur on each individual site where both accounts have a profile also. Merges always go into the profile that has the lower user ID, and whatever display name, profile picture, and other information that is in use on that lower-ID profile is what gets kept after the merge unless that information hasn't been changed yet on that profile.
Since your profile with the lower user ID still used the default display name, and your other profile had a display name explicitly set, the display name was copied over as a part of the merge.