Cursor horizontal position is memorized by the browser.
| is cursor position
Step 1: Put cursor here.
1234567890| 123456
Step 2: Press down key.
1234567890 123456|
Step 3: Press up key.
1234567890| 123456
Otherwise, if you put cursor directly here, the cursor will go up to the same index of the row.
Step 1: Put cursor here:
1234567890 123456|
Step 2: Press up key.
123456|7890 123456
1234567890
123456