LOL...actually I was getting set to PASTE that graph in to help explain it!! It quite nicely reinforces what I said. So lets review what I said the 20 hour measurement actually measures: The maximum total amount of STEADY current that the battery can produce for 20 hours before going flat.
Notice NO mention of 20 amps of current.
OK now lets look at your graph which is a 20 hour rated group 31 battery rated at 100 amp hours.
Now look at the left side where it says AH capacity and see what VERTICAL line is crossed ... read down that line and find the continuous amp draw required to get 100 amp hours in 20 hours. It is 5.
So 5 amps current run for 20 hours gives us a capacity of 100amp hours at the 20 hour rating.
Now just for fun...lets look across at the result from drawing 20 amps...and you can see you would get just 63 amphours total capacity before the battery was flat. So you can also figure out that 20 amps of current only allowed the battery to LAST a little over 3 hours at 20 amps per hour.
The 20 amp hour rating method is of little value in and of itself since we don't run steady state loads in real life. But it does give a meaningful comparison about the current loads a battery will support for 20 hours before going flat. You cannot compare ANY other amp load rating with the 20 hour rating as you can see from your battery curve...batt's can last longer with lower loads..and a lot shorter with higher loads.
Whenever you see a battery amp hour rating at the 20 hour spec...simply divide by 20 to see the maximum continuous amps you can run for 20 hours staight when new.
BTW...your pdf attachment verifies this with their analysis of a 70 amp hour battery...quoting:
"For example, an average automotive battery might have a capacity of about 70 amp-hours, specified at a current of 3.5 amps. This means that the amount of time this battery could continuously supply a current of 3.5 amps to a load would be 20 hours (70 amp-hours / 3.5 amps)."
So 20 hours = 70 AMP HOURS/3.5 amps.
Transposing we get 20 hours x 3.5 amps = 70 amp hours.
Notice no mention of 20 amps in the purple either.
Hope that makes it more clear...
EDIT...was typing this as you wrote you last reply...you figured it out without me but maybe this post will help others.