it is to do where the pit box is in relation to the start/finish.
team exiting the pits fits new tyres and cross the line the tyres are 1 lap old
team passes the line to fit new tyre they have done 13 and not the 12 when they pitted
Thanks for the bug report Gustavo. This is a known issue. I believe James' description of the problem is accurate. At the moment the tyre history counter does not take into account where the pit box is in relation to the start/finish line. At the moment I can't give an ETA for a bugfix but it is on my radar.
