Problem with Multiple Calendars and Float

Study the following activity table and Gant chart, all the necessary information should be there. The calendar for activity A has continuous working days. The calendar for activities B and C is for a 10 days on 4 days off roster.

problem_with_multiple_calendars_and_float_a.png

problem_with_multiple_calendars_and_float_b.png

Note that the end date will not be pushed out unless over 2 days duration is added to activity A. This means that activity A should have 2 days float.

Why then does activity A have a float of negative 2 days?

(hopefully the picture isn't too small)