The reason was that you never change the value of var_one and var_two, so the line "var_display=..." always calculated 5-2 in game, if you want to have it further reduced (and don't need to keep the original value of var_one), you don't even need var_display and modify var_one directly, like:
at the top of room/global script:
//you've already done it
int var_one=5;
int var_two=2;
// comment out declaration of var_display, wont hurt and it's not used any more
// int value_display=0;
then somewhere in script:
value_one = var_one - var_two; // just like add
//displaying it on a GUI
//we need a string
string str_Value;
//converting an int value to string
StrFormat(str_Value, "%d", value_one);
//showing it on a GUI named STAT, label number 0:
SetLabelText(STAT, 0, str_Value);
at the top of room/global script:
//you've already done it
int var_one=5;
int var_two=2;
// comment out declaration of var_display, wont hurt and it's not used any more
// int value_display=0;
then somewhere in script:
value_one = var_one - var_two; // just like add
//displaying it on a GUI
//we need a string
string str_Value;
//converting an int value to string
StrFormat(str_Value, "%d", value_one);
//showing it on a GUI named STAT, label number 0:
SetLabelText(STAT, 0, str_Value);