I have posted before about this exact same problem. No explanation yet....
I noticed that GPS tracklogs (from a Garmin 76CSx) were appearing in Memory Map slightly west of where they should be. In Google Earth and on Garmin's UK Topo map they were spot on.
Try this test:-
Put a waypoint on a grid line intersection and manually type in the exact correct grid reference. Of course this should put the waypoint bang on where the grid lines cross. My official 25k Memory Map of the Lake District (off a DVD) fails this test. The waypoint appears slightly off. It was the error I'd noticed.
It appears as if the relevant Memory Map qct file is not calibrated accurately enough. Also the error varies slightly (only a few metres) over the area that the map covers.
Here's an actual example.
A waypoint placed dead on the intersection of the two grid lines immediately ESE of Scafell Pike's summit shows a grid ref of NY 22019 07007. NOT the nice round numbers you'd expect.
A waypoint with the correct grid line intersection grid reference of NY 22000 07000 is incorrectly positioned 19m off to the west and 7m to the south of where it should be.
I've uploaded two images which show the two waypoints on the map and also the information box about the map being used.https://picasaweb.google.com/geoffmozz/ ... -Ip-mCmwE#
I'm currently using the map with a datum shift of 4 metres North and 13 metres East. This is close enough for the bit of the map I'm on right now.
I shall be interested to hear why the maps are slightly out. I guess many others have the same issue, but because the error is small may never have noticed.