The overall measurement is not a good measurement to use as a comparison between loaded cartridges. I know its all confusing but rather than just reloading and shooting your reloads, you are asking questions some reloaders never ask until they have been doing it for years.
The correct measurement method is to use a bullet nut (comparator):
This device allows a comparative measurement between cartridges but uses the bullet Ogive rather than the end of the bullet. The seating die does not seat the bullet by pushing on the bullet end but by pushing on the bullet Ogive (part of the nose of the bullet). So every bullet is seated the same from the bullet Ogive to the case head.
You set up the OAL on a test cartridge per the book spec (or other more advanced methods that you don't need to know about just yet) using the OAL. Then you slip the bullet nose into the nut in the hole for the caliber of interest (30 cal for 308) and measure from the other side of the nut to the cartridge base with calipers. You record that number and all cartridges should match that number when measured this way. Any ammo made in the future should be gaged the same way and made to match the same number for this bullet.
The way bullets are formed, any excess material ends up protruding into the end of the bullet nose. Every bullet is the same except they might end up with small differences of the excess materials in the end of the nose. In a box of bullets you'll find big variations in the length of the bullet but very small variations when using the comparator nut. If the bullets were made perfect you could not afford to buy them! This way gives the most accurate bullet for the least cost. Accuracy is not affected by this manufacutring process normally.