1. The specifications are not well defined; and 
2. The rating can vary significantly between vendors and camera types.

The lux ratings can vary significantly because:

• There is no standard test and measurement method for digital cameras. While there are standards for analog cameras, these do not generally apply to digital cameras.
• Analog cameras are measured at fixed video rates of 30 or 25 FPS at video resolution. Digital cameras have much wider varieties of resolutions and frame rates (or exposure times).
• Most testing is done with visible white light and does not show the sensitivity outside the visible spectrum or at a specific wavelength.
• Gain of the cameras can vary significantly between camera models. This makes it very difficult to compared the 0 dB gain or maximum gain of cameras even when they use the same sensor.

Sensitivity is generally defined as enough light to "get a decent picture". The specification is the aperture size for a given lens with given illumination (e.g 400 lux for monochrome cameras or 2000 lux for color cameras). The camera gain is usually minimized to reduce the noise in the image. The minimal or nominal gain can vary significantly, making this measure difficult to compare across vendors or camera models.

Minimum illumination is generally defined as enough light to "get a picture". The specification is the illumination in lux to get a picture. The camera gain is typically set to maximum with maximum lens aperture. Camera gain can vary significantly between camera models, the exposure time is usually not specified, and the undefined definition of a "minimally acceptable picture" makes it difficult to compare this specification across vendors.

The only reliable way to compare performance of multiple cameras is by testing the cameras in the final imaging environment. It should be possible to compare the published specifications of multiple cameras from one vendor since the same test method is probably applied to all models. However, comparison of the published camera specifications from different vendors is very risky.