Among other things, Christoph Hormann wrote:

>> (higher = brighter)
>> Venus: 0.76
>> Earth: 0.33
>>  Moon: 0.12
>> 
>> The Earth is 21% brighter than the moon (as you've said), probably
>> because of all the water on the surface and in the clouds. Venus is 43%
>> brighter [...]
> 
> If earth reflects a fraction of 0.33 of the incoming light and the moon
> 0.12 that means the earth surface reflects 2.75 times the amount of
> light the moon surface reflects per surface area.  I don't even want to
> try to imagine how you came to '21% brighter'...

The moon reflects 12%
The earth reflects 33%

33-12 = 21

The earth reflects 21% more than the moon. Now, 21% is a relative measure, 
so the question is 21% of what? Daniel meant 21% of the whole original 
light income, while you're thinking of 21% of the amount of light reflected 
by the moon (which, obviously, is not correct in this case, as you say).

Talking about percents can easily be ambiguous. We often hear in the news 
things like "this year the inflation has been 0.5% higher than last 
year's", meaning that it was 1% last year and 1.5% this year (and not 
1.005%).

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