I noticed that the cabin chimes on the Boeing 747-400 overall sound one semitone lower than on other aircraft, including the 737, 777, Airbus A320, and even the 747-8. The same semitone-lower chimes are used on the Bombardier Q400, but that was built later.
Notice here that with the exception of some Qatar Airways planes using the "high" chime instead of the "low" chime for the seat belt sign, all the "low" chimes sound the same, with the exception of the Bombardier Q400. That chime is the same as the one on the Boeing 747-400.
Why is this the case?
Update: I noticed that this question was marked as being primarily based on opinions. However, it's not based on opinions. If you looked at the video links I supplied above, it is a fact that the cabin chimes on the 747-400 are lower-pitched than most other planes. All of the planes in the compilation video, except one, are at the same higher pitch. It's not just my opinion that they're lower-pitched. Also, if the answer is that "the chime manufacturer simply built it that way", that should be an answer. Questions shouldn't be closed based on the answer, the merits of the question alone should be judged.