Getting sore when switching bikes

I di most of my recent training on my XC bike (just training outdoor).
Occasionaly I took my trail bike, to shake things a bit up.
Now every time I did that, my legs felt very sore next day. Even after yesterday, where I did a very easy 1h endurace ride as part of my ongoing recovery week.

I though that must be due do a difference in relative position of the contact points. So today I sat myself on each bike next to a wall and used a color marker to mark the position of the pedal at 3 o’clock, the knee as well as the hip joint. By sitting on the bike I could eliminate the influence of suspension on the effective geometry.
To my own surprise, the three mentioned points matched perfectly within the limits of precision this method implies.
I didn’t mark the handlebar as it was obvious that this would differ, sitting higher on the trail bike.

Now I wonder what could be the reason I feel more sore after a workout on the trail bike?

  • Maybe it was just coincidence and I need to confirm it few more times
  • The geometry of the 3 contact points matched on flat ground. My workouts are all on steep uphill, maybe there the seat of the trail bike slips a tick more behind as the rear suspension sags in. But I think I would just compensate that by sliding forward on the saddle, alone for the reason of performance.
  • Maybe the influence of the handle bar position is bigger that I thought? It should allow a bigger hip angle on the trail compared to the XC bike.

I wonder if someone experienced something similar and maybe has a solution to avoid the issue.