That surprises me too. Almost every seatpost I've ever measured, including long MTB types, have the minimum insertion mark about 7.5 -8.0 cm above the lower end. Having the end below the toptube/seattube intersection is important too but you report you are OK there.

I just measured several Easton and Trek house brand seatposts in my parts box in both 27.2 and 31.6 mm. All are MTB length and all have the minimum insertion 75 - 80 mm above the end. The one odd exception is an Easton EA30 (their cheapest, heaviest model) in 31.6 mm and the minimum insertion is 100 mm above the end.