This is exactly what I’m experiencing with my 9kw Daikin. Heat punk estimates circa 5-6kw heat loss but I’d like the 8kw so it has additional overhead but can modulate down to the same minimum as the 6kw. During the January cold snap when it averaged -4 degrees overnight and -5.9 at the coldest point the heat loss was ~5kw.
I believe the root cause having spoken to Octopus is assumptions / default air change rates which don’t reflect reality.