Global Variables dont get proper values if any operation is done on them with values that are not whole numbers

Was trying to make a GUI Level-Up system yesterday that allows you to change your stats when I noticed that the global variables that I  had set to add/subtract values with numbers that are not whole number's started giving wrong results after a few operations. For example a number that is supposed to increment by 0.5 every button press gave me values like 6.000004 and on subtraction went down to E+ values and below 0 even when  I set an if condition to not decrease value if its already 0.