I presume you knew the 5.4 test result was either static or on the up otherwise I would think its a bit low to start on bike journey and it not alarming at 15.4 for a one off, its better than other way. I say this as I rode for many years and you need good readings as there are not second chances and there are plenty of split second thinking on bikes.
Is it becaise it is all motorway riding with not much body movement country rodes to me take a bit more energy as you are throwing the bike around more and the blood is pumping more. You should not be getting many adrenaline rushes if you are used to riding and the bikes limits unless you ride on the edge constantly but could be cause, I still have the bike but it's not as much fun day to day riding with speed camera, traffic calming and stupid junctions that come out to the centre of the road.