OK, let me ask a few more ...
#1 - What am I actually affecting by adjusting the tappet clearance and is there any benefit to going outside of the recommended specs?
Allowing the engine to breathe properly, the only reason for going outside the recommended clearances would be if different camshafts were fitted.
#2 - What causes each valve tappet to be of different sizes in the first place?
Tolerances, even from new can vary widely, even blueprinted engines still differ.
#3 - I assume that wear and tear would cause the tappet clearance to increase over time, not to decrease. So am I right in assuming that because valve's 1 & 4 have only half the required clearance 0.203 & 0.254 respectively that they were not done correctly in the first place?
Wear and tear are the main factor but not in the way you mean. As long as the cam lobes aren't worn (case hardening still intact) then nothing really changes. The shims don't wear as such nor do the tappets as long as the oil/filter are changed regularly using recommended or higher spec fully synthetic lubricants.
Although noisy tappets are the result of either wear on the cam lobes, cam bearings, tappets etc. making the gaps wider as things gradually wear. The reason for the gaps being smaller is probably due to the valve heads wearing into the valve seats in the cylinder head after opening and being slammed shut by the valve springs many millions of times. This effectively moves them closer to camshaft hence the reduced clearance.