[jaclaz]

I can confirm, the 28C actually shows the 4 (viewable) registries of the stack, numbered on the left as:

4:

3:

2:

1:

So, if you input 4 [ENTER], 3 [ENTER], 2 [ENTER], 1 [ENTER] you have:

4:4

3:3

2:2

1:1

But if you input 4 [ENTER], 3 [ENTER], 2 [ENTER], 1 you have:

3:4

2:3

1:2

1

if you then press +, you get 3 in both cases, which is the sum of the bottom two lines in the display.

Those are either the same as the "old" x and y registers or the sum of last input and the bottom register, same as the old x.