I've touched this earlier but now I want to know more.

I have noted that on some Win7 systems (maybe x64 is a clue?) some of KM accels disappear from the menu and do not work.

I copy working portable Twin with all accels working to the new PC - and voila! - some are now missed.

Now I have the next hotkeys disappeared:
Ctrl+S
Ctrl+N
Ctrl+T
Ctrl+O
Ctrl+G (Google it)
Ctrl+Alt+Z (reopen closed tab)
Ctrl+Alt+S
Ctrl+Alt+N

while some others of that 'type' are in place
Ctrl+X
Ctrl+C
Ctrl+V
CTRL ALT VK_PLUS
CTRL ALT VK_ADD
CTRL ALT VK_MINUS
CTRL ALT VK_SUBTRACT

and so on...

I can't understand 4 things:
1) why they disappear?
2) what is the determining factor of what systems are exposed to this (am I right that only Win 7 x64?)
3) what principle of choosing accels for 'switching off' is used?
4) Is there sense and possibility to make 'Seven-safe' accel-config?
For example, I've replaced Ctrl+Alt+Z (undo) with old Win 3.11 hotkey Alt+Backspace and it works!