No, let me break it down to better explain.
If you play 1 tier you have a 40/60 on that tier
If you play 2 tiers you have a 40/60 on each one of those tiers
If you play 3 tiers you have 40/60 on each one.
All that means is that for example if you play 10 games in a single tier in theory you should have 4 that it will be on your tier and 6 that can be up-tiered. If you play 10 in 2 tiers it is possible that all 10 will be up-tier because you would need to play 20 (10 on each) to have in theory 12 up-tier and 8 same tier. Obviously these numbers are affected by the time of day you play and what you are playing. So mathematically speaking if you only play 10 matches your chances to get up-tiered is greater for every different tier you play. Keep in mind that this does extend to other days so that`s why sometimes you might get a feeling that the MM is being nicer to you, just means you could have hit that 40% in every tier that day.
I missed @Skpstr answer. Thanks.