Any question answered...

Athlete setting on the Tanita scales, yes

Could be hydration mass. I weigh myself pretty much the same time each morning so naturally dehydrated from over night fast.

Muscle mass.. Hmm.. If I lost any more of that I really would look like a stick.. So I hope not.. I have been doing 1-2 big gear hill reps sessions per week. Not high HR but slow, steady, good form "grinds" to build strength and, hopefully, some more muscle mass.

To be fair it is only a few %. Dropped a kilo (65kg down to 64) this week and BF is now 10% rather than 7% at 65..

Just interested how this is really

Then prob ignore it, and monitor long-term trend (month to month). Obviously, the lower your BF%, the more the reading varies due to other factors - like water composition. You may be losing a bit of upper body perhaps-ensure you do some light trunk chest and arms work, not least to preserve core stab. Then see how it goes.