The main volume isn't one of the function settings that get preserved when the keyboard is turned off. Only 16 function settings are preserved; they're listed on page 32 of the PSR-E433's owner's manual. The other 47 function settings, plus all the panel settings that aren't listed in the function menu, are set to their default values when the keyboard is powered up.
The main volume and other voice-related functions-- including the functions related to the reverb, chorus, harmony, and arpeggio-- are set to specific defaults according to the main voice that's been selected. The exceptions are the voice number and function settings related to the split voice-- they're changed if you select one of the "Left" arpeggio voices (i.e., voices 260 through 269), but otherwise are left unchanged when you change the main voice.
As it happens, the main volume-- the volume for the main voice-- has a default setting of 122 for voice 001, which is the voice number that the keyboard defaults to when you power it up.
Fortunately, there is a way to set the main volume and save it-- by saving the panel settings to one of the 32 registrations (i.e., 8 banks of 4 registrations). Unfortunately, when you select a different voice number for the main voice, the main volume will get changed to the default for that particular voice-- so if you don't like the default main volume for that voice, you'll need to change it and then save it as another registration.