Because the different instruments have different timbres, this can camouflage itself within the same pitch and slightly fool the ear if you're not really listening. Now, you may be thinking... "Of course I can tell if things are playing in unison or playing in harmony. Why can't you?"
I can, but when you're not playing close attention, things are moving by so fast and you're not really listening for it, I'm saying, yeah, you can miss it. Especially if it's spaced out by a few octaves with the different instruments. I may sound like harmony even though it's not.
Remember, there are overtones and undertones at play here. These will go a long way towards the harmonics.
Also, keep in mind that while the instruments have their own timbres, the players don't all sound the same either! I'm a trumpet player, so we'll go with them. Here's a picture depicting 3 trumpets playing together. They're the same in every way, except for the separate players styles or sound. Let's say, one has a clear tone, the other has a raspy tone, and the third a fat blat-type of tone. That would give the same note three distinct sounds and fool the ear slightly.
You can see they're all the same, just their color is different.
Now, let's look at some different instruments.
Let's say this is a trumpet, saxophone, and trombone playing in unison. See that the different tone qualities make for a unique sound with each instrument. If we displace the sound by an octave or two, how much more would our ears be fooled momentarily by the depth of sound.
The simplicity of this on the part of the arrangers is genius. We can create sounds that bring the full representation of harmony without actually writing it. Sometimes, if written well and spaced correctly by the different instruments, our minds will create the missing pieces all on their own!
"Wait, are you saying my brain will actually fill in the harmony if I hear tones played between an octave or two?"
Yes! I am.
Now, don't get me wrong. We definitely can't do without writing harmony. If this technique is used too often, the ear catches on and it gets boring. But if used sparingly and wisely, this simple technique can enhance the quality of music.
As new arrangers, we tend to overdue it. Thinking we need to fill every section with tons of harmony and all of the instruments playing at the same time. But this is a mistake.
Follow the K.I.S.S. rule. It will save you from yourself.
Deep breaths. We're getting there. Chugging up the mountain. I think I can. I think I can....