I think the difference is, launching the car at the track as normal, you have max converter slip for a short time and it's coupled as close as it will be by the top of low gear.

Now take the same car and launch/make a pass in high gear. Would be a ton more slip/heat (resulting in less power to the tires) and to the same effect, would also show on a chassis dyno.

On cars with multi plate lockup converters, can often be a huge difference between locked and unlocked numbers.

Or, going from a less to more efficient converter with no other changes can result in much higher numbers with no other changes.

“Dyno Disappointment”

Actually......... not at all uncommon.

Sort of similar to...... “first time at the track disappointment”

Torque converters can make so much difference in the car performance that is is hard to believe until you test several different ones or see someone do that at the track.
the last converter swap I did was from a custom built 10 inch street and strip Contintental to a 8 inch SS Hemi Turbo action, that swap made my car pickup .2 ET in the 1/8 mile and gained 2.0 MPH
We've seen .8 ET gain and very little MPH loss going from a stock 11 inch street Hemi converter to a custom built 8 inch Rossi race converter in the 1/4 mile
The bad side of different brands is also possible, GER were junk The first 8 inch race converter we bought and used was a Art Carr 8 inch race converter for Hemi NHRA stocker, it would slow the car down 3.0+ MPH and .3 ET in the 1/4 mile from the stock 11 inch street Hemi converter