The RPN stack only contains 4 numbers (named X, Y, Z and T). Your first keystroke sequence tries to push 5 numbers onto the stack, which results in the top number (8111) getting pushed off the top and lost. As you key in each number, and after you press [ENTER} or any operation key, visualize in your mind what each stack register contains.
One way of performing your calculation without losing the 8111 off the top of the stack is by keying 6 [1/x] instead of 1 [ENTER] 6 [ / ].
The correct answer is 114.44, not 144.44.
Your final keystroke sequence gets a correct answer because the first 8000 gets lost off the top of the stack but it's replaced by the second 8000 because it's the same number. The T register replicates itself as the stack drops.
It may help to read the section in your manual about how the RPN stack works.
Disclaimer: I don't work for HP. I'm just another happy HP calculator user.