Don't know the official answer but here is a technical one. When you combine two frequencies you get those two frequencies out in addition to the sum and the difference of the two. For example, say you combine a 300 and 700 Hz tone. What you'll hear is 300, 700, 400, and 1000 Hz. So my guess is because you generate 4 times the audible range with 2 frequencies.
Looking at your other posts, I see we are talking about horns for a 69. Numbers on the horns (signifying how the mounting bracket is shaped and positioned) and types of horns (dual-tone vs. single) vary for each of the 1st gen years.
All 69 Camaros had the "dual-tone" horns, with part numbers ending in 289 and 290.
The replacements you have probably have a plastic horn body and a plated mounting bracket that can be positioned as needed. The replacement horns were meant to generally sound the same as original, but GM did not bother to source replacements in the numerous mounting bracket configurations used as OE on the various GM models.
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