LED strip brightness and its measurement - did I get ripped off?
You're measuring the Illuminance and comparing it to the Luminous Emittance - two very different things.

Illuminance is the amount of light that falls on your sensor.

Luminous Emittance is the amount of light sent out by the LED.

You have to remember that the light emitted from the LED is sent out over a cone of 120°, so the closer your sensor is to the LED the more of the light will fall on it, and the less will miss it.

At the moment you're measuring a small portion of the actual light - just what shines direct on your sensor, and a small amount reflected by the inner surface of your "box".

You'd achieve the best measurement if you could ensure that 100% of the emitted light fell on 100% of the surface of your sensor, and the only real way of doing that would be to have a sensor the same dimensions as the point of light in the center of the LED.