Working Time changes cause unexpected results

Your situation stems from misunderstanding how the calendars work. In general when you create a task it will assign a "day" as what the project calendar says is a day and also the calendar options (tools, options, calendar). Then when a resource is assigned the assignment looks at the resource availability on the resource calendar and the schedule is controlled by the resource calendar after that. UNLESS - there is a task calendar with override of the resource calendar flagged.

So now you have a situation where the task was created at 6.4 hours = a day and the resource says something else and the calendar options could be saying something else and you have a mess on your hands. This is why the calendar and options should be set up before the first task is entered. If you had changed the calendar after the tasks (and the assignments) are in, you get things even more goofed up. The fact that the work number is changing at times leads me to believe you are also using fixed duration tasks. The amount of work would be tied to the amount of time a resource has available within a time period. You could also be using effort-driven when you shouldn't. You might also be plugging dates and creating contraints.

So what is your solution? Create a new project with the correct calendar settings and with the resources set up correctly and import in just the task names and the relationships. Set up your task types and effort-driven flags per task. Then start entering work and/or duration per task. And then start creating assignments. sorry, no easy fix for this one.