The difference is in the height of the sight above the line of the bore. A scope is typically twice as high as irons.
With a scope mounted close to the bore, say 1 1/2 " above, I zero at 25 yards. I do that for two reasons, 1. I'm sure to be on the paper, and 2. After only one shot, adjust my cross hairs to bisect the hole from the first shot.
Now I can move out the the 100 yard line and shoot a 3 shot group, expecting the bullets to land about 3" high. I can refine my aim at this range and be pretty sure it is sighted in as well as I can aim.
Depending on the velocity of the round, and its ballistic properties I can figure where it is zeroed. i.e., a .270 with a 130 grain bullet at 3100 fps, should pass zero again at 275 yards and end up 4" low at 325. At apogee, the bullet will be 4" high at someplace near 175 yards. Shooting at those ranges ought to confirm this.
If we slow the bullet's muzzle velocity down to 2900 fps, we will lose 25 yards from the second zero and 4" low i.e. zero at 250 -4" at 300.
For every 200 fps more or less muzzle velocity expect 25 yards more or less on these figures. This is called the point blank range sighting. Any place from the muzzle of my rifle to that magical far away place the trajectory will put the bullet within 4" of the line of sight, so I can ignore range for the most part, put the cross-hairs 1/3 of the way from the brisket up toward the back, definitely behind the front shoulder and kill the game animal.
As you can see the so called flat shooters don't gain a whole lot on the moderates with this method. If I start a 129 grain bullet from my .260 at 2800 fps, it takes a real rip snorter of a magnum to send a bullet of similar ballistic properties at 3200 fps and gain a whole 50 yards point blank range on me. At what cost in barrel life, at what risk of shooting the internals of the scope loose, at what cost to your hearing, or your wallet? You'll have to buy the $2 bullets, and half again as much powder, all for a measly 50 yards extra range. Provided of course you shoot enough to be able to use that 50 yards.
Iron sights should be started a 12.5 yards, you can expect something like 2" high at 100 yards 0 at 150 and 5" low at 200 yards. I can't see well enough to get all the range out of that cartridge without some sort of scope sight. With the scope 0 for a 2100 fps muzzle velocity, 5 increments of 25 yards less than the .270 or 150 yards. Hey I already said that! The difference I can now use the 200 yard effective range of the old turdy turdy.
Some people prefer the 200 yard zero, claiming the game at 300 looks so far away, they just can resist aiming high and shooting over. I can understand that philosophy, but using the duplex reticule in my variable scope I can avoid gross errors in range.