Distant to Empty calculation is based of your average MPG calculation. Before you topped off the tank you had a decent average MPG because you had driven a decent number of miles on the freeway and/or conservatively round town.
After you topped off the tank, your average MPG reset and in the short time from driving out of the gas station to where ever you looked at the Distance calc, your average MPG was significantly less than just before you topped off.
Since the amount of gas you put in the car was not significant (I assume by "topped off" to mean a couple gallons), the decrease in average MPG from the short trip out of the gas station out weighed the increase in mileage from the extra couple gallons of gas.
Therefore, your Distant to Empty calculation was less than it was before.
The only possible explanation always seems plausible.
If you had continued to monitor DTE after top-off, you probably would have seen DTE increase. e.g. After city running, I find DTE is 600km, but after a little while on highway cruise, it climbs to about 800km.
Actually No. DTE at top off was 720 kms. After Top off, DTE was 640kms. Just drove 70 highway kms and am just under 570 kms DTE. Gas needle still on FULL. After Top Up, I should have been over 800 kms DTE. Totally befuddled!