On a treadmill, why is the degree of incline shown on the machine not the true incline?

When we run on an incline, in addition to running forward, we are moving upward against the force of gravity. Our legs are performing work by lifting our body mass up – like walking up stairs. The work is defined as force against gravity times the distance.

On a treadmill you are basically running at a fixed height. To simulate the extra load you would be facing on an incline, the internal mechanism gradually puts more load on the rollers.. If that were to happen quickly you would suddenly run forward. To further add realism the platform tilts a certain amount to make you think you are on a real world incline. It also helps make your feet and ankles bend as if you were.

But…. Making a moving incline with the required actuators is expensive and theoretically unnecessary since it is easy to simulate the load, So, as an engineering/marketing compromise, they give you a little lift and add load to the drive system.
Don’t worry. Even though you are not actually going up an incline your body is still doing the same workout.

Incline or no, the device will still work as a perfect clothes rack 4 months after you get it.

Can’t improve much on @LuckyGuy‘s full & impressive answer, but only to add that a treadmill can’t replicate running up a prolonged, steep incline with a strong headwind.
That, people of the internet, is bloody hard work but extremely rewarding nonetheless.

@LuckyGuy So, if I want to simulate going up a 20 degree hill, I need to set the machine at 10 because that is the tilt of the machine, plus the machine is simulating an additional 10 degrees by putting more load on the rollers? Is that what you’re saying?

@2davidc8 No. If you want to simulate a 20 degree hill use the setting they recommend. The platform could be at 10 degrees or it could be at 20. That does not matter. What matters is the load or resistance the machine puts on the rollers. If you tell the machine you are at 20 it will put more resistance on the mechanism so you are burning something like 500 watts instead of 400 watts.. (Those numbers are crude estimates. They depend upon your weight, BMI, and speed and probably other factors.).

@LuckyGuy I’m having a hard time understanding how the machine could be imposing additional resistance because if I just stand on it, the belt would just run me off the machine without my doing anything. Can you please explain?

Sure. First, I am assuming you have a fancier unit. Correct?
If it is, then load is electrical and is a function of how fast the tread is moving. There is an electrical generator that adds and subtracts load. There is a controllable electric field that can be varied to make the generator work harder – or not at all. The treadmill also knows when yo are standing on it. Some even weigh you! They use that info to program the load as well.
When youl are standing still the load is almost nothing so the tread moves easily.

I use the machines at the gym; I don’t know how fancy they are, but they have settings for both incline and speed. If I want to use up more calories per hour, I can either increase the incline or increase the speed or both. They both make me sweat more and increase my heart rate more.
But the speed setting just makes the belt go faster, so that I have to run faster to keep up, but it doesn’t seem to have anything to do with “load”. Increasing the incline setting just increases the tilt so it makes it seem more like running up a hill, but again, I don’t see how the machine is adding or subtracting “load”. So, my original question was, if I set the incline at 10 degrees, is the angle physically truly 10 degrees, or is it just a relative machine number, meaning a 10 makes you work harder than a 5.

Maybe take a protractor with you, the next time you visit the gym?
That’s what I would do.
Can’t get it any clearer than seeing it with your own eyes.