HModify needs to lock the record before changing its value.
If record wasn’t locked previously by an explicit "Read and lock", WinDev will read and lock the record, compare its actual value with the value it had last time current program read it.
If value was changed (by another user), WinDev showup a dialog box, and ask the user what to do.
With WebDev, this dialog box is not showed, and an error is raised. You will have to manage this error case with HOnError, or you can set DRVOPTIMISTIC Connection property in Extended info.