Incorrect Distance Recorded

I have recently replaced my Garmin Etrex 10 with Etrex 30x

I usually use it for hikes recorded on my computer.   These have been recorded on the Etrex 10.

Before the hike I delete all information held on the garmin and download the new route.  

At the end of each hike we usually compare distance and ascent, and there is often a small difference between us

However lately my recorded distance has been way out.   Yesterday I did a hike of approximately 12km, but it was recorded as 23.73 km

When I downloaded the date to Basecamp the track and waymarks were correct, but the distance was shown as 23.73km

Any help or suggestions would be much appreciated