
There are a few factors which can cause this -
1) Tyre pressures - Dont trust that gauge at the servo

- Get yourself a good gauge and test it out against others until you trust it

. Get a compressor at home to make checking your tyres damn easy. Check your pressures before every trip. Tyre's are porous and will lose pressure when just sitting in the shed. Its amazing how much tread gets ripped off when pressures are low. Try a few more PSI to account for your pillion, gear on the bike, winter gear on you adds weight, camping gear, trailer if you are towing etc.
2) How is your bike loaded? - Do you have water bottles or wet weather gear in one pannier only leaving the other side empty - Loading makes a difference
3) Wind - A cross wind for half a day from one side will cause you to push into the wind to keep the bike straight wearing that side tread down more

4) Road camber - You don't get it so much in the city, but get out on the country roads, and depending on the camber of the road, you can lose a lot of tread

Picture the ruts in the tar where the heavy truck wheels are - If you sit on the right side of the wheel rut you will wear the right side of the tyre down, but if you sit in the left side of tyre rut in the middle of the road you will balance the camber out and wear the tyre evenly - It takes practice to find the even camber part of the road which means more riding

5) Road surface - different types of tar to stone ratios will mean different wear - I.e. Hot mix to stone top country roads
6) Tyre quality - Different tyres give different performance - Word of mouth will set you straight - I haven't ever heard someone get a lot of kms from a Bridgstone where the tyre pressures weren't right for the loading on the bike
Just a few things to consider. Hope that helps
