Hi Russell, what's your ideal end result here? Understanding what's happening or aligning your Strava data so it more closely resembles actual distance travelled?
A quick-ish fix to the latter is to use the split function within Strava to break your ride into 3 separate activities: pre-break, break, post-break, and then to delete the messy break activity. You'll now have two activities rather than one, but their total distance will be more accurate. You can download and combine the gpx files with a bit of extra work if having a single activity is important enough to you to warrant the effort.
For the former...
Much as others have said, to a certain extent what you're seeing is just how GPS data are and different software and settings will have differing effects on how much the numbers gets tidied up. Other variables to add into the mix include satellite configuration in relation to the receiver and environmental structures at the time of the reading, orientation of the GPS module, whether your body is between the receiver and the transmitting satellites and, even, whether there is dew on a nearby buddleia bush or not!
I've done extensive work collecting gps data with 'identical' receivers, and they never give the same results - each has their own character it seems.
Looks like a nice spot for sarnies, anyway!