Keith, I think it may be a matter of semantics mostly, but the 20-hour mention is the common time frame usually referenced for rating batteries, according to the Battery Council. Peukert’s Law notwithstanding! The 20-hour time frame is simply the usual benchmark.
Here's how one major battery manufacturer states it:
"The amp-hr rating for an automotive battery is usually based on a 20hr discharge at 80° F, at a particular rate (determined by the amount of active material within the battery), until a minimum voltage of 1.75V/cell (10.5 volts for a 12 volt battery) is reached. For example, if a 12-volt battery at 80° F delivers 4 amps for 20 hours before its voltage drops below 10.5 volts, its amp-hr capacity will be 4 amperes x 20 hours = 80 ampere-hours."
Scientific measurements/data will be different. Obviously, to be 100% correct, the Peukert formula would have to be factored in. But to explain that would simply further confuse most readers not holding a degree in engineering.