From a deco perspective that makes very little sense.
Shallower stops are always going to be longer than deeper ones. Doing deco on air at 20 feet will take longer than it would at 10 feet.
My guess is the safety stop depth is set at 15 feet so the computer decides deco stops need to be deeper ??
Safety stops are optional,not mandatory. What is the status of a safety stop after a deco stop? i.e. is it really a safety stop or a deco stop??
I suspect, the manufacturer set the last deco stop to 20', (I can think of several reasons but that is for some other time,) so they can put their safety stop in there too.
In my computer I have the choice to set my last stop at 10' or 20', but then again it's not the typical recreational dive computer
. I have it set to 10' but usually opt to stay at 20' or go to 15'.
In the PADI/DSAT Tec courses, a safety stop was never mandatory, but it was optional. If your last stop was at 10', then you extend that stop by 3 minutes. But this is not to be confused with Deep Stops, PADI/DSAT always recommended deep stops, 1/2 way between your bottom depth, and first deco stop for 1 minute.
I don't know of an agency that recommends using a computer only in technical diving. I remember doing a lot of planning, and cutting tables for my dives in the PADI/DSAT. Our tables were always more conservative than the computers, because we typically did not stay at the bottom depth for the entire time, so the computer always showed a shorter deco times. However, if something happened where your computer exceed your table, then you had to clear your computer.
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Cause when I do choose to do a computer based deco profile if I am one inch above the stop for 60 seconds, my computer will refer me to tables (some of my computers will lock me out). Therefore to keep the computer alive, I do the rounding by being a touch below the stop.
I know of 1 computer that has said "refer to tables," if the diver blew a stop, if I owned that computer, I would sell immediately after that dive. Typically a dive computer should be the backup to your tables in technical diving.