A wheelchair ramp spec, a road grade sign, and a level's degree reading can all describe the exact same slope - genuinely useful to convert between them correctly, since a ramp built to the wrong number is a real accessibility and safety problem.
A ratio like "1:12" describes rise over run directly - genuinely useful for construction, since it tells you exactly how much horizontal ramp you need per inch of height. Percentage grade is the same math expressed differently (rise/run x 100) and is what road signs and most spec sheets use. Degrees measure the actual angle from horizontal, which is what a digital level or angle finder reports. All three are legitimate ways to describe an incline - the trap is assuming a "12" in one system means the same thing as a "12" in another, when a 1:12 ramp and a 12-degree ramp are dramatically different steepness (12 degrees is roughly a 1:4.7 ratio, more than twice as steep as ADA allows).