That's because because brightness and intensity are two totally different animals.
Just because something is BRIGHT, does not mean that it is INTENSE.
Gather around my children, it's learning time!
Brightness involves how we as humans perceive light and it's NOT a physical quantity. Some familiar units of measurements are lux, lumens, and candelas.
Lux is the measurement for the amount of visible light spread over a surface area, also known as illuminance.
Lumens or luminosity is the measurement for the amount of visible light emitted by a light source. Lumens are a standard unit that can be used to compare different light sources.
Candelas are of measurement for the brightness of light based on the human eye's perception, more specifically a measurement of directional intensity. Candelas replaced the term "candlepower" in the late 40's (they had a slightly different conversion formula back then, but today it's relatively a straight 1:1 ratio between candelas and candlepower.) A light source has a luminosity of one candle if it appears to match the output of a standard candle.
Intensity is the power per unit area and IS a physical quantity.
When growing cannabis there are specifics units of measurement we're looking for when it comes light intensity.
In cultivation, this is defined by PAR light.
PAR (which stands for Photosynthetically Active Radiation) is light of wavelengths 400-700 nm and is the portion of the light spectrum utilized by plants for photosynthesis.
We quantify (or to put in a measurable form) PAR light using PPFD (Photosynthetic Photon Flux Density), which is defined as the photon flux density of PAR.
PPFD is measured in micro-mols per second (μmol/s), indicating how many photons in this spectral range fall on the plant each second.
The PPFD of the full moon is about 0.01 micro E / m(squared)s while middle of the day sunshine readings average 2,000 micro E / m(squared)s. Moonlight at full moon equals only 1/200,000th sunlight at noon.
So yes, you can read a book by moonlight for sure, or you can see your inside living room with even a tiny LED light being on with your TV and electronics, but while they are BRIGHT to our SENSITIVE human eyeballs, they are NOT INTENSE, meaning they are providing an extremely low (negligible) PPFD, or light intensity, that would actually support photosynthetic growth.
So smaller light leaks are less concerning, but as the source of light becomes more INTENSE, that is what is going to potentially causes stress in the plants.