The way it shows most is coins where the wear is more obvious. Pre-1920 large silver rarely has more than half a grade difference between obverse and reverse. The shallow portraits - most obvious on halfcrowns - are often encountered with more than a grade difference. For example, the obverse not even F but with a reverse hovering around the VF mark.
Yes, between 1914 and 1918 it can be difficult to find fully struck up examples due to the huge increase in mintages during WW1, but it's not hard to find examples in EF especially as many were hoarded. However, I'd amend my original comment to say "harder to find the shallow portrait in high grades" rather than strictly UNC - you only have to look at that portrait and it begins to wear.
Agreed - and all the way to 1926, maybe even 1927 too. But the ones I meant are a small number of pennies where there are flecks of yellow (brass?) long after the lustre has gone. These metal flecks are actually embedded in the metal of the flan and give the coin a very odd appearance.