This query falls awkwardly between "Enlarging" and "General Exposure", but as the main aspect specifically concerns darkroom exposure, I've put it here.

I use mostly roll film and while I hold the Zone System "in mind" when taking photographs (in the sense of being mindful of where I'm expecting my tones to "fall"), I'm not a "scientific" photographer, and any one roll is likely to have a wide variety of lighting and contrast scenes on it.

But I'm trying to improve the consistency and printability of my negatives, and I've read Barry Thornton's "unZone" article a few times, and find it largely clear and potentially very helpful (http://www.awh-imaging.co.uk/barrythornton/unzone.htm).

On p.4, he says the following:

Quote Originally Posted by Barry Thornton
... using this minimum time for maximum black exposure, make your full contact sheet processed in the standard way. Be careful how you give the exposure. If, say, you found the time on the test strip to be 5 two second steps, do not give a ten second contact sheet exposure. This will give significantly greater exposure. Give the same 5 two second bursts of light.

Now I think I understand why he might say this (is it to do with the way the paper will respond as if being "flashed"?) - but I am likely to be wrong in this, and if so would like to understand better what's going on here.

So, in sum - why might a single 10-second exposure give greater exposure than 5 2-second exposures?