Assuming it was a 12L tank, it looks like around 2min 29s were excluded from the total time. It means using the time 40:30 instead of the 42:59 shown on your screenshot, for the overall average. And the exclusion of the 2½ minutes would cause an increase of the overall average:
And that seems to be corresponding to the drop to zero on the SAC graph for about that time. Did you use another tank, held your breath, surfaced, was there a connectivity problem, or what was the reason for that?
Please note that I am not telling I agree with the method of calculating the average, excluding zero consumption periods, I am just explaining why the discrepancy might have happened.
... that told, even the calculation using the full time would still result in the avg consumption of 37.13 l/min, hence there would still be an important discrepancy between the real-time data, and the overall results. Is the tank size properly set in the transceiver settings?
So to me it looks like there is some problem with your tank volume settings. If you post a link to the activity as shown on the address bar when you open the activity in Garmin Connect Web, I could perhaps guess better where the problem is.
I change the size tank yesterday (no define to 16l) but there are no changes about the SAC on the app.
I am afraid you have to do it before the dive. The data coming from the transmitter is based on the parameters available in the real-time. Try another dive and see whether it helped.
BTW, did you change the tank volume in the transmitter settings too, or just in the activity details? Without correct tank data, the transmitter cannot give you correct data during the dive.
In the hidden background data of your activity, it looks like the volume of the tank was set to 12 litres because the data shows the total consumption of 1594 litres at the pressure change from 291.23 to 158.29 bars, which almost exactly corresponds to 12 × (291.23-158.29) = 1595.28.
So I really recommend setting it up correctly and trying another dive.
... and since you use MK3i, also make sure your firmware is up-to-date (current version is 17.32, and it brought a bugfix for incorrect avg SAC values). However, the correct tank volume in the transmitter settings is still necessary anyway.
