Softpoweroff is not standby. It doesn't behave differently from Lock keys & screen (other than letting you activate it with a single keypress—I use a short-press on the power button—and allowing you to specific the offline mode behavior).

Yes, this is why there's no standby mode, because when things aren't eating CPU, the tablet is effectively using no power (you can go about 30 days from full to empty with an idle tablet, and about 10-14 in a more average case). Unfortunately this doesn't work out in all cases. Toss in WiFi and you're down to about 4 days, toss in broken WiFi and make it about 24 hours. Adding some process taking 1% CPU and you're down to about a day, combine that with a broken router and you don't get much battery life at all.

Thankfully, though, idling works just great for most users.