为什么lea会添加不同的结果？

Question

你能帮助我吗，我有这个ams代码：

; subs two long number
;    rsi -- address of minuend (long number)
;    rdi -- address of subtrahend (long number)
;    rcx -- length of long numbers in qwords
; result:
;    sub is written to rdi
sub_long_long:
                push            rdi
                push            rsi
                push            rcx

                clc
.loop:
                mov             rax, [rsi]
                lea             rsi, [rsi + 8]
                sbb             rax, [rdi]
                mov             [rdi], rax
                lea             rdi, [rdi + 8]
                dec             rcx
                jnz             .loop

                pop             rcx
                pop             rsi
                pop             rdi
                ret

现在，如果我将lea rsi, [rsi + 8]更改为add rsi, 8程序会给出错误的结果，例如：

10000000000000000000000000000000000000000000000000000000000000000000 1000000000000000000000000000 9999999999999999999999999999999999999999000000018446744073709551616

为什么会这样？