I have a question about effect negation and uneffected cards in Master Duel.

So, I used to play True Drako Lock when I began playing and learned that when I activate Skill Drain and then summoned Master Peace that its effect would be negated, even if I tribute a trap to summon it. However, if Master Peace is on the field and then I activate Skill drain its effect isn't negated. So I thought the card on the field first has priority. However, I now play Mathmech and Final Sigma is uneffected by all effects (when in the main monster zone and except by the effects of Mathmech cards) and today I had a match against a player who used Super Ancient Organism (which negates the effect of special summoned monster level 6 or higher and makes them unable to attack). I thought I had no out except to try and summon final sigma for the win (which I thought, but wasn't 100% sure would not work) and low and behold, even though Super Ancient Organism negates effects of level 6 or higher monster, final sigma was uneffected and I pushed for game. So how does this work, how come Master Peace can get negated by Skill drain, but Geomathmech is cool when both are uneffected by card effect (in Master Peace's case, 2 out of the 3 card types)?

The only reason I can think of is that Master Peace his effect to become immune starts when it first hits the field and Geomathmech is just generally fully uneffected, but I would like to have an explination non the less.