Hello, I will explain it to you:
With the release of Windows 8, one of the changes in how is system handling booting was made.
In Windows 7, when the bios loaded bootloader (pretty small program designed to start Windows OS), and there were multiple entries, it would create a menu for you to select which is you want to load. Because of its small size, the menu was completely text based and controlled only with keyboard.
With introduction of Windows 8, the part for selecting which OS to boot, was moved "up", closer to the Windows core, and in the bootloader, only "load the default OS" part was left.
So what is happening now, is:
Bootloader will directly start loading core files of the default OS, but it'll left a message for the OS it's loading, that there are multiple choices.
Windows core, loaded by the bootloader, then starts loading Windows as usual, but just before it gets to the logon screen it'll show you the blue "beautiful" graphic screen with mouse, keyboard, options and animations support. Just because files and drivers that support advanced graphics are already loaded, it can render that better looking UI selection screen. So Microsoft engineers, just because they could, (nah, someone from UX was surely pushing them) they made it do so.
This is also a reason why picking the default OS on that screen brings you to the logon screen so fast, because basically half of the OS is already loaded (in the RAM).
But you decided you want to boot Windows 7 this time! But there is half of Windows 8 loaded in the RAM already! Oh crap, what now?!
Well the safest, dirtiest/cleanest solution for this case is to just reboot, to completely clear RAM, and start booting from beginning again. But just before reboot, Windows on the selection screen marks a temporary override of the default OS (at next start, boot once to Windows 7) onto the drive, so the bootloader doesn't load the files of Windows 8 again.
So you select the Windows 7, PC reboots, main bootloader starts Windows 7 bootloader, but leave message to the W7 bootloader that temp override is active, just so W7 bootloader doesn't ask you again (in the old text screen style) which is to boot and proceeds straight to loading Windows 7.
Wow, that was more than I expected to get out of me...
Anyways the point I'm wanting to make is, that by selecting W7 as default, the old, W7 text screen style bootloader will ask you which OS do you want to start (and because it first asks then loads the OS core, it doesn't need to reboot PC regardless of which os you choose here, unlike the new one, that, if you won't pick the default option, it has to throw away half of loaded default Windows files)
What I recommend you to try is:
Download program easyBCD, in the bootloader entries configuration, select W8/8.1/10 as your default OS, and then UNcheck the "use metro style bootloader" where the "metro" (that's like that solid color blue window theme) means new with mouse support (yeah, I hate that metro one) and the old (unchecked metro style) means just use text screen selector.
I hope that'll help you, but mainly, that you've learned something today
