Funnily enough Jo I've just been pondering this same thing. I suspect it's an inaccuracy with Garmin's GPS.

I had a Forerunner 220 until Christmas when I got a Forerunner 235; In December I ran a route with the old watch, and it measured up at 4.10 miles; I've just run it again with the new watch and it's only 3.80! To lose .30 miles over just 4 miles!? It's crazy.

I blame Garmin, but it'd be great if GRG could somehow "rightsize".

Sorry not to reply to this post earlier.  We will have a look into it, but the info comes straight from Garmin so not sure if we can do anything about this.  You can edit a route to tweak it to the correct distance by going into the route through the 'my saved routes page' but not directly in the log book as you say Jo, as then there would be a discrepancy between the route in your saved routes and the one in your log book.  

I was just about to ask a question about this when I saw this thread. The distance is usually similar on here and with my garmin (if it's a run I haven't measured on here and logged) but the big dicrepancy I get is with the time - GRG always has my average pace as faster than my garmin stats. I suppose even a small discrepancy in the distance logged might account for this - but I don't know which one to believe. Think I'll go with GRG as the stats are more favourable!