Getting the correct comparison between your finished rounds and the comparator has everything to do with sizing.
The OAL gage is NOT measuring from the head to the ogive. The modified case is pushed into the chamber until its SHOULDER contacts the chamber shoulder.
You then push the bullet forward until it touches the rifling. There are only two fixed points, the case shoulder and the ogive. Unless your cases measure the same distance from the head to the shoulder as the modified case, they will not exactly match the gage reading. Since the gage is sized to SAAMI minimum, your properly sized cases will almost never match the modified case, unless your gun has a SAAMI minimum chamber.
In reference to the OALs mentioned in the OP, he is talking about the ogive reading being 2.953, resulting in a final OAL of 3.508.