Explanation re the larger GD than expected from the 36/37 match

FamilyTree DNA have replied to my query about why the apparent discrepancies between the Genetic Distance (GD) values that you'd count from the 36/37 marker match (GD 1) being reported on the personal pages as GD 3 (this between Doug (F-6) and David (F-7).

The correspondence is below for you to interpret yourself, but my interpretation is that we can take 2 off the GD counts on the personal pages because the mutations are isolated, given how well the rest of the markers match.
I also extrapolated that to the GDs being reported with the rest of Lineage 1 (Martin is shown as GD 6 and Earl and Roy as GD 7), so this would bring them back to 4 and 5 respectively?
Although given the differences are in a range of markers shown as having a faster mutation rate, I'd like to see how this all pans out at 67 markers.
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From Darren (FT DNA):

The reason this is being thrown off is that we see a genetic distance of 3 happening at CDYb,. David has a 35 where as Douglas has an 18. However this is likely only a genetic distance of on with one the 35 from CDY a copying over the 38 of CDYb, so instead of 15 18, we see 15 15 in one mutation event. If we were to see three individual mutations at this single marker we would expect to see a lot more mutations happening at other markers. So we can likely treat this as a single mutation event and reduce the genetic distance by two to all of his matches at the 37 marker level, based on this single marker. We do not yet have a system implemented to show these types of mutations in the database so far right now it will continue to be read as a genetic distance of three when comparing the two men. We are seeking to improve this.
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-----Original Message-----
From: FAIRBAIRN DNA
Sent: Monday, November 24, 2008 6:38 PM
To: darren
Subject: GD numbers?

Hi Darren,

I'm puzzled.

From my FAIRBAIRN GAP pages, clicking on the GD Report for David Fairbairn, to see what his newest set of results do to the GD reports, I get (at 37 markers):

https://www.familytreedna.com/gdrules_37.html
Name Distance
Doug Fairbairn 3
Martin Fairbairn (Y67) 6
G. Roy Fairbairn (Y67) 7
Earl Fairburn (Y67) 7


The GD info at
https://www.familytreedna.com/gdrules_37.html

shows a GD 3 as 34/37, BUT

the results show that
kit 131048 is a 36/37 match.
And the GD 6 and 7 above are 34/37 and 33/37 by my calcs.

What am I missing?

Lorna