Why am I getting different values based on order of input when the task type remains the same?

Question

I'm having a hard time understanding the results I'm getting when I input task information. Here are the scenarios with different, unlinked fixed work tasks:
I estimate a task will take 2 hrs to complete and put this into work. If I add 1 resource, work = 2 hrs, duration = 1 day?, Resource A = 25%
On the same task, if I add Resource B (along with keeping A), duration = 0.2 days, Resource A remains 25% and Resource B = 100% (no % shown)
If I try the same scenario (adding first resource, then adding a second) on another task, occasionally it will give a duration of 0.91 days??? What the hell is that about?
If I add 2 hrs for work then add both Resource A and Resource B at the same time I get duration = 0.5 days, Resource A = 25% and Resource B = 100%. Why is it giving a duration of 0.5 days and why does it not adjust the unit % for Resource B?
This one is really blowing my mind. If I don't add work but instead add both resources at the same time first it's either making work 8.2 hrs OR 16 hours! I've gotten this result on the same exact task doing the same exact input. How can this differ?!
No work input, both resources added initially and IF I get the correct 16 hours with 1 day duration, then I go ahead and change work to 2 hours, which produce Resource A & B = 100% and duration = 0.13 days.
This last scenario is the duration result I want to see so the resources can work on multiple tasks in 1 day, but I typically try to input the work or duration estimates into the schedule and then resources last. I also don't understand how there can be
such a wide range of results based on such similar scenarios. Also, having to do it this way creates another step since I have to go back and change the work...although I guess I would be doing that if I input estimates first.
Can anyone explain why I'm getting those very head scratching differences, as well as provide some insight into how you initially develop a schedule (order of inputs)? I'm wondering if this has something to do with the change in 2010 regarding resources
that I still don't understand?

Answer 1

Thank you Julie for the quick reply. As indicated in the first part of my question all these scenarios are dealing with Fixed Work, so yes by default they are effort driven. I should have clarified on the 1st scenario the question mark was just to show duration
had not changed, I wasn't actually questioning why it hadn't changed. Scenario 1 worked as expected. Although I don't fully agree with it, Scenario 2 also worked as expected.
Scenario 3: Yes it is effort driven (fixed work). Resources are just test/fake resources so all information, including calendars, is default. This is one of those cases where I can produce that unusual result only on occasion, not everytime.
4. In my scenario it's fixed work, but I do get the same result as you with fixed unit. Question still remains in both cases of why is the duration 0.5 days on 2 hours total work? Also, why are both units 25% in fixed units, but in fixed work Resource A
is 25% and Resource B is 100% when in both cases they are working 1 hour? Lastly, if Units is going to be changed in this case then why isn't it 13% (1/8 hours)?
5. This is also fixed work and I'm not touching duration in any of the scenarios. Day is still default at 8 hours. I was assigning resources via Resource Names on the Gantt View. I'm thinking this had to be some sort of glitch because I can't consistently
produce the 8.2 hours result, but it happened multiple times.
I also follow the same steps you do, other than I guess the last one. When you say you enter work estimates while assigning does that mean you do that in the split pane/task details form? I've typically done it from the Gantt View, but that seems to be what's
causing my issue because when I tried doing it from task details it is giving me the duration I want. So thank you for that. I still don't understand why there is such a wide range of results, but at least it makes more sense to me that 100% Unit working on
a task duration of 0.13 days is 1 hour.
I apologize for how long this, but I'm trying to get out of some bad habits I've picked up by being forced to develop dumbed down schedules for the past few years.