RTP of Reel Power slots from Aristocrat

Hi all,

I have a question about Reel Power slots from Aristocrat (e.g. Imperial House).
My concern is why when you play only with 1 full reel, bet multiplier is 1.
2 full reels - multiplier 5.
3 full reels - multiplier 10.
4 full reels - multiplier 20.
5 full reels - multiplier 30.

It seems to me that 2 full reels should have multiplier 3 (number of ways to win is 9 instead of 3),
and 3 full reels should have multiplier 9 (number of ways to win is 27 instead of 3, chances of scatter are increased accordingly).
I understand that multipliers for 4 and 5 reels should be calculated using more complicated formulas,
but about first 3 reels I am pretty sure that simple multiplication by 3 and 9 should be done.

Could anyone explain me why they use these strange multipliers?
It seems that it causes different RTP for different reels number selected.

Thanks in advance,
Anton.