Why are task numbers different in the HTML and the Gantt chart view?

Tasks (and milestones) in i-lign have a unique number that is assigned to them when they are created and which always stays the same. Its purpose is to give people a shorthand way of referring to tasks. The sequence is simple, starting at one for the first task created in a project, then two, three etc. If a task is deleted its number is not reassigned, so the sequence may come to have gaps. Users can reorder tasks, so the number does not correspond with the order in which they appear in lists. Finally, the number is prefixed by the project number if one exists, so can be unique across projects.

The gantt chart does not show the task number, but does have an 'Id' column. Its purpose is to make it easier for people to work with constraints. The Id shows the current position of the task or milestone in the list, so a task's Id will change if it is resequenced. This makes it easier to find a task by Id in a long list, especially if the project contains summary tasks. A task's constraints are shown in the 'Pre' column and also on the 'Constraints' panel. Finding these constraining tasks by Id is easier if the tasks are sorted in Id order.