What makes this a problem is a few makers like DPMS, ArmaLite and Bushmaster put the shorter A2 FSB on their flat tops then sell you a rear sight that's also shorter to correct it. The difference in height is 0.04" I believe. But many people buy 3rd party rear sights which tend to be designed for the taller FSB. Bushmaster does sell a taller front sight post you can screw into an A2 FSB to correct the problem.
Actually Colt is to blame for this problem, they should have used the A2 FSB for both A2 and A4 uppers and just shaved down the A4 carry handle 0.04". Then 3rd party would have followed Colt's lead and we wouldn't have this minor headache.
It's not a big deal as long as you understand why you can't zero with the iron sights you bought. The Bushmaster fix is only about $6 but of course all this doesn't apply to you, RC.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek