Grade maths

Trying to understand the grade maths on the traverses on this boulder.

For the original traverse Kani (7C+) adding the traverse into the start of Ninja Gaeshi (7A) adds 5 grades. But add exactly the same traverse onto the start of Mushi (8A) and it only adds one grade, Kani-Mushi (8A+) 🤷

I feel like we could do with some local knowledge, tvhc_tvhc I don't suppose you know the problems in question?

I’ve done Mushi and Ninja Gaeshi up which the traverses finish. Eyeballed the traversey bit and watched people on it.