Gee, you'd think that anyone that smart would have just plugged in the 15A plug to the 20A recp. unless twisting the prong is what gives you the additional 5 Amps
the receptacle that the 3 way tap was plugged into was origionally a 240 volt rececptacle. A 120v 20A receptacle has the other prong horizontal.
if guessing that origionally this was hooked up as a 220V outlet for a 220V water treatment system. somewhere down the line, someone installed a new system and rather than wiring properly, they just grounded one leg and twisted one prong so they could fit the plug into the outlet.
Trumpy, I've seen Aussie/NZ/Chinese plugs with the pins twisted so that they're parallel (ground pin hack-sawed off) so they could be jammed into American type sockets.