5v isn't inherently provided by the vehicle so the ECUs have 5v and 3.3v regulators or DC-DC converters in them to step down the 12v to 5v and/or 3.3v needed for the computer electronics - those systems expect a certain voltage above 5v to work. As the vehicle is powered on there is often a surge in demand from other systems which pulls the 12v rails down well below 8v if you have a bad battery which affects the reset circuits on the computers and sensor reading, which is why you often get spurious error codes seemingly unrelated to the actual problem.
The RX400h, being an older generation vehicle and hybrid system, seems to cope better with a lower battery voltage than say an IS300h but it is unpredictable. As well as the brake booster, you have auto lights that could come on if it isn't bright outside when you turn the key, the auto wipers may move if there is condensation on the screen, the steering wheel will start moving as soon as the key touches the ignition barrel - all of this makes it difficult to predict if you will be able to get into 'Ready' mode when your aux battery is at a certain standing voltage - and it depends on the battery condition and ability to hold a charge, not just voltage.