Could be that they were not set to spec in the first place. Better to loose than to tight. All parts wear. Cam lobe or wear on the follower would result in increase clearances. The valves live in a pretty harsh environment. High temps, high RPM plus containing basically an explosion causes them to wear also. The valve seat is usually forced into the engine block over time, all would decrease clearances. So it could go either way, but typically they decrease and fall within the range of normal, unless it happens in a short period of time or the engine loses compression. I do not know if there is some peculiarity to this design, but in principal it appears very similar to my TDM 850 with its shim on the valve stem and the bucket being placed over it for the cam to ride on. The clearances always got smaller. Sounds like it wears like that design, as there was very little movement of clearances.