The Mac Mini and Studio are due for an update in the coming months, a part of this is also probably that they’d rather save memory to build up their next gen model inventory rather than current gen ones?

Like trvz said, they use different memory. M3 Ultra uses LPDDR5X 6400 MT/s, M4 Max uses LPDDR5X 8533 MT/s, while all the M5 models use LPDDR5X 9600 MT/s.

No, the memory is different enough.