Saddle creep

There is a lot of backwards force going into a saddle while the rider sits on it cranking out the kilometres. The saddle on my Madone, I discovered yesterday, had slid backwards a good 15mm since I first assembled it last year.
This bike has a proprietary seatpost (like all the good Treks at the moment, but a post unique to the Madone) which separates rotation from retention. There are two bolts, one for each function. Retention includes the fore-aft adjustment. With other Trek (Bontrager) posts you have to tighten the single bolt in the seatpost head really tight (well above torque spec). Looks like that’s the case for this one too.
I tightened it with a torque wrench and still it moved.

Incidentally, putting the seat back where it belongs made an improvement to the bike.

I’m more used to saddles creeping downwards from a poor post/frame interface (or simply not tight enough binder bolt). I thought I’d fixed my Single Speed from downwards creep by adding in a soda-can shim. But today I discovered that the saddle was 20mm low. It was low enough that it felt really low. Having restored it and tightened it with a torque wrench, I wonder how quickly it will drop again…