Total Float not equal in same relationship

All,

I am trying to understand in what way oracle primavera calculate toatal float in same logic relationship. Thereby, I made a sample testing projest with only 3 activities A B and C

A--(FS)-->B--(FS)-->C

The duration for A B and C are all 5 days.

Now comes two results for total float with different calenders

Situation-1

When I use the same calendar(7days and 10hours) for A B and C the total float are all "0", which seems to be reseanable.

Situation-2

However, after I change calendar for B into "5 days 8 hours" then F9, I got 2.6 days for A and 0 day for B C.

The defult setting is 8 hours per day in primavera( Just for information I am not sure whether it is in relation with total float calculation)

Could anybody explain how does primavera calculate total float? why I got 2.6 day total float in situation-2?

Thank you guys.

Dennis