You are absolutely right. Of course lockup doesn't affect the gear ratio. If we ignore lockup in my figures the revs drop from 2400 to 1850 due to the overdrive gear ratio alone. This is about 0.77:1. I thought the gear ratio of the 418RH was 0.69:1. This didn't make sense.
I then calculated the ratio between RPM in top against that with overdrive and lockup and that came out at 0.68:1. This led me to question whether the stated figure wasn't the gear ratio but an overall ratio.
Once lockup in gages there's a 0.01 difference between the stated and calculated gear ratio right ?
Your question why this difference, my answer probably tire size used in calculation.
Are you using actual measured tire 2x radius (center of axle to ground) or calculated tire size?