P6.1; WBS Total Original Duration Problem

WBS title is Item #1 (included Activity #1 & 2)
1.Activity #1; Total Float =0 and original duration is (8days) start 04-May-10 (8:00) finish 12-May-10 (16:00)
2.Activity #2 will start after finish activity #1; Total Float = 33 and original duration is (1days) start 13-May-10 (8:00) finish 13-May-10 (16:00)

After run schedule I found the following:
WBS title is Item #1, Total Float = 27 instead of 33 and original duration is 8days instead of 9days start 04-May-10 (8:00) finish 13-May-10 (16:00)

I wonder why this conflict happened.

Please advise by steps.
Best Regards,
Abeer