system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: UV Index - How is it measured?

"**The UV index is defined as the effective irradiance on a horizontal surface..."**  
I bought a UV Index meter, and the measurements don't match the definition above. When I put it on the surface level, the measured UV index is lower than the reported. I can only get the reported value when I point the device directly to the sun! This completely makes sense to me, because otherwise, the reported UVI would be much lower to vertical objects (aka humans) than the realistic one when the sun is at a low angle, and vice versa. Does anyone know what is the truth?

EDIT: stop upvoting, the answer was too obvious, aka the question was stupid. Big thanks to /nalc!
gpt
The horizontal surface in this definition is the horizontal surface of your UV sensor, with horizontal meaning that it's pointed directly up at the sun and not at an angle towards the sun. 

Pointing it at objects will essentially measure UV emissions from the sun times the reflectivity of the surface (which is not necessarily the same in the UV as it is visually). So unless it's a really shiny mirror it will be less.