I had the out of line steering wheel and it turned out tracking had been done by adjusting the track rod on one side only. The steering wheel will only go on in the expected straight ahead position so it important that each track rod end is adjusted equally. So tracking should have cured it if done properly. If its increasing it does sound like a tyre issue and as its increasing that would indicate something progressive and would worry me. Perhaps its time to swap tyres around to see if the pull changes with the tyres. And yes , I have heard of tyres causing issues like this.
Citroens since 1969
Currently DS4 2.0hdi 180bhp EAT6