You have assigned Joe at 20% of 4 hours per day (his calendar) which is .8 hours per day. You have assigned Bob at 80% of 6 hours per day (his calendar) which is 4.8 hours per day. If you want Bob to work 6 hours per day and Joe to work 4 hours per day, assign them at 100%.

I get 6.5 days calculated with a task start of July 31 at 8:00 am and a finish of August 13 at 12:00 pm. Bob is working 32 hours (80% of 40) and finishes at 12:00 pm on August 8 - and Joe is work 8 hours (20% of 40) and finishes on August 13 at 12:00 pm.

In File, Options what is your definition of "hours per day"".