One way to look at it is that it's less than perfectly circular.

It does indeed lose material and can no longer seat properly - properly defined as on the seat the full 360 degrees of seat circumference.

Most times, the exhaust valve burns.
ID'd by mild backfiring/popping when the fresh fuel/air mix gets ignited early.

Leaking intake valves make an engine run rough - as does the burned exhaust valve - but doesn't do the backfiring bit.

Easy to understand why exhaust valves burn when you realize the normal operating temperature for them is red-hot.

Lack of time on the seat - so as to transfer heat - is usually what burns an exhaust valve.

Too many valve grinds on an exh valve make it too thin and create the potential for a burned valve.

If your valve job is fresh and the valves weren't too thin - auto shop machinists will usually reject thin valves so you're probably ok there - you may want to take a look at valve adjustment.
And check compression somewhere along the way.