You are overanalyzing the problem. No way valves are going to wear that much in a couple of thousand miles. And, if you have lash on your exhaust valves you know that isn't the problem. So it's possible your measuring technique is incorrect or that something is basically wrong with the engine, like camshaft put in wrong and bent a couple of valves.

A leakdown test like RR said would be a good place to start. And no, a couple of thousandths on the lash won't make a whole lot of difference on a compression check.

R.