The timer uses units of time that can be considered "seconds" but are actual exposure units which vary in time length. With the Zone VI Compensating Timer and RH Designs StopClock Vario, each "second" is marked by an audible tick. As the cold light tube increases its output, the ticks become shorter. With the old single tube Zone VI head, the increase in speed was dramatic. If the light to the cell is is too bright by virtue of its being very close to the light source, the timer ticks will be very close together making the numbers inconvenient. The Zone VI Compensating Timer (IIRC) had some adjustment to reduce the effect of strong light, and the RH unit has also. See my post above in which I describe my sensor being outside the head and just looking through a hole at the very bright VCL4500 tubes (pointing more directly at the blue tube to ensure that it isn't swamped by the green tube which is on all the time.

Richard's suggestion to black out part of the sensor achieves something similar.