What might be the reason for redacting a blank line ?
Notwithstanding acorngen's post about the redaction 'strips' being applied manually, I understood that they were computer generated using an algorithm which attempted to follow the line regardless of optical curves in the page and/or misalignment between the left and right pages per image. Hence some of the 'strips' having a dogleg in them, and not always in the correct place. (Iain...'s Folkestone examples being classic cases where the algorithm got it completely wrong).
In which case, a blank line would have no year of birth, and with no year of birth the algorithm would not know whether or not that line should be redacted and therefore
should logically redact it by default.
I've seen various cases of blank lines - or cases where the birth year was missing or mistranscribed - which were redacted in the original release, but have been unredacted now.
I don't know as fact this is how the redaction process worked, but have a feeling I read something to that effect. Furthermore, it would make more sense that these errors were made by some software AI process which wasn't 100% accurate rather than them being made by a human applying a physical 'strip' over the original document before the image was captured. I don't think anyone could justify how a human could come up with the result in Iain...'s example, unless the human was deliberately making gross mistakes.