When using justification for fonts with variable widths, such as Helvetica, should the characters shift position when characters are changed to the same width characters?

For example,

1) Create a rectangle immediately beneath the text I/O object
2) Set the text IO justify mode as 6,
3) Send string “1909” to text I/O
4) Send string “1910” to text I/O

I saw the text shift position slightly -- even characters 0 to 9, which have the same width in Helvetica font.

The characters will not shift if the widths are identical. Even a single pixel difference can cause a shift.