This only works if you are adding 2 numbers together. Try adding 2+2+2 and you still get 4. The enter key is basically a key to clear what is already there and enter the first number. You then enter the second number and press the function you want. For example, 2+2+2 would be.
2 enter
2 +
2 +
The same is true of any math function. 5 enter, 5 enter x will give you 25. 5 enter 5 enter, 5 enter x will still give you 25 while 5 enter 5x5x will give you 125.