On 2016-05-16, The Natural Philosopher wrote:

It looks pretty clear to me. Originally there was no bathroom,. just a
kitchen with a ceiling and possibly a flat roof.

Then the bathroom was added, and the ceiling joists were not at the
right height, and or inadequate. so a new set of joist were laid in to
take the floor

AFAICT, the ground floor kitchen & 1st floor bathroom were built at
the same time as one offshot (on a mid-terrace).

I would say the evidence is against that, personally.

Definitely. The masonry shows the kitchen & bathroom were built together.

Or it could be that the builder at the time had ordered 6x3 timber, was
under pressure, and realised he needed 7x3, to match the existing house,
and said 'sod that, I'll use two sets of joists instead and 6x3 timber'
or whatever.

That could be it!

BTW WTDF is an 'offshot' bathroom?
One what isn't in video view?

It's an extension, narrower than the rest of the house, behind a
terraced house.