It is called reseting pre-boot credentials. BIOS is using this information to speed up initialising internal components instead of gathering it from slow internal serial bus on every boot. BIOS is also constantly updating this information with its own meassurements and statistics (storing its own settings to improve stability).
This information can be corrupted or obsolete. By example when CPU is overheating (due to thermal paste deterioration), such statistic help to set power management parameters. System becomes less responsive, but continue to boot. It is why it is important to reset learned credentials after refreshing thermal paste and cleaning air ducts, upgrading CPU, internal LCD display, etc...
Reseting CMOS defaults in setup prograsm do not clear pre-boot credentials. Removing CMOS battery does. Holding power button helps to drain power from internal capacitors in 5 seconds instead of hours.
Question is why reseting pre-boot credentials happens without removing CMOS battery? I don't agree with the argument of oveloading CMOS battery, so voltage drops below limit. You can meassure it easily using multimeter - it doesn't happen...
There are CMOS latches controling ACPI power state in the startup circuit powered by CMOS battery. Hardware shutdown (by holding power button for 5 second) is controlled by the same latches. So my educated quess is that when you perform hardware reset with no power applied it is latched and on next power-on BIOS will detect such condition and ignore pre-boot credentials (at least on some systems).