Add to that the fact that the effective photo active radiation (PAR) that lands on the plants throughout the year vary significantly as the seasons change.
When it comes to light, there are two main factors that need to be considered:
1. Maximum solar energy that the plant can tolerate. In most cases it is not the intense illumination that kills orchids, but the intense UV rays that piggybacks along with it.
2. Minimum PAR that the plant needs in order to perform photosynthesis - assuming the plant has access to water during the photoperiod.
The light ratings given by vendors for orchids are based on their growing conditions and what works best for the plant based on their experience. So the rating they give could actually be anywhere between the upper and lower end of the plant's natural range.
Barring special seasonal triggers, a plant receiving a consistent supply of light towards it's low-end limit could get the same results as a plant that gets fluctuating light supply that dips over and under it's natural range throughout the year.
Think of this simplified formula (although it might be more art than science)
Energy produced = (light intensity + water) x Photoperiod
To compensate for low light intensity, increase the photoperiod.
However, you can only extend the photoperiod so much before you end up depriving the plant of the dark phase that is also crucial for plant development.