Question about Stack and Roll

[Antlab]

Hi all.
I have a question for the RPN experts.
To refresh my RPN skills, I am re-reading the original manual of the HP-15C, the Owner's Handbook. I dont' have the original HP-15C, but several Android emulators.
The question is this:
Page 34 of the manual reports the simple example that fills the four registers with the digits 1, 2, 3 and 4.
The normal method is obviously:
1) 1
2) Enter
3) 2
4) Enter
5) 3
6) Enter
7) 4

At this point, with the Roll Down key is possible to inspect and verify the contents of the registers.

My problem is that if I use Roll Down to inspect the register contents after each Enter of the previous sequence (returning each time to the X register) I found at the end that the registers are filled with the sequence:
T: 2.0000
Z: 3.0000
Y: 3.0000
X: 4.0000

This surprises me, because I was expecting the sequence 1, 2, 3, 4.
Probably I am missing some subtlety of the RPN Roll behaviuor, could you please help me to understand what happens?
Thanks.