Every session in attendance is a "grading event" - if you have 10 sessions and the max grade of every session is 10.5, then the total possible "score" from within the attendance activity would be 105.
A good example where decimals might make more sense is that you might give 10 points for each "Present" value, but a value of 9.5 if they are marked down as attending "5 Minutes late" or "8.5" if they are 15 min late.
The summary at the bottom of the page shown in your top screenshot gives a breakdown on how the sessions are scored within the attendance activity and how the overall score within the attendance for a student is calculated.
the gradebook sits "outside" the attendance activity and is used for calculating final grades in the course. a good example of this is a course like this:
quiz - worth 30% of the total grade (maximum point level in the activity set to 30)
Assignment 1 worth 30% of the total grade (max point level in the activity set to 30)
Assignment 2 worth 20% of the total grade (max point level in the activity set to 20)
Attendance worth 20% of the total grade (max point level in the activity set to 20)
so... a studen that gets:
90% in Quiz 1 gets 27 points for this activity in the gradebook
80% in assignment 1 gets 24 points for this activity in the gradebook
60% in assignment 2 gets 12 points for this activity in the gradebook
50% in attendance gets 10 points for this activity in the gradebook.
if you add those up, they get a total points score of (27 + 24 +12 +10) = 73
Obviously that's just one method of how this value is used - there are other much more complicated setups as the gradebook is extremely flexible.
Hopefully that helps - but if you have further questions about how the gradebook works you might be better creating a new post in the gradebook forum: