RPM Airgap

I would assume nothing, this is not factory stock. There are several reason why a different than stock length could or would be used, and even different lengths within the same block. Once you start milling things, changing rocker ratio's, bad valve servicing, really crappy shade tree - make it work - bad mechanic mentality. You can run in to all kinds of things. I know you kept track of which rod came out of which hole, a long with which lifter if your putting this all back together with the same parts? If you were running a different ratio on the intake than the exhaust then I suppose you could run different lengths for each set of rockers to correct geometry over the tip. How many of each type is in this motor?