1. From the top 300 or so modern cards, maybe 50 are dual lands, all rares; and maybe close to 100 more are rares. That means that for the rare slot Modern Masters will have to exclude rare dual lands and rares of other types that are acknowledged modern staples. It also means that if they want to keep the number of dual lands same as in other sets, there will have to be some serious exclusion of dual land staples.
2. Only a few planeswalkers make it into the modern staple group. If there are three planeswalkers in this set, the third one could be a stretch as a modern staple.
3. (others have mentioned) Some of the modern staples are one in a group where the others did not make the staple classification-e.g. swords.
4. Modern staples get displaced as more sets are added to modern. The modern staples right after M13 are not the same as the ones now. RTR has displaced some of these, and so will Gatecrash. As this set hits and then M14 comes out some of the modern staples in this set will be "recently displaced" modern staples.
From a sample of online and paper card tournament top decks I came up with 7,934 cards and picked the 330 cards that were played with at least a playset when combining all of them. This means that if in all of these decks a card was included in decks and sideboards three or fewer times, I excluded it. We know that sets with many dual lands, such as Zendikar and Innistrad, did not have 44 dual lands. In the Modern Masters set will as an inclusion constraints there will have to be a good number of staple dual lands that do not make the set.
The color balance of these Modern staples is reasonable, with the exception of a smaller number of black cards.
Some sets fare better than others. "S" means that the card is a Time Spiral Timeshifted "Special" card. The early blocks have good proportions of Common, Uncommon, and Rare.
Its only with the more recent sets that the Rare and Mythic Rare imbalance shows up, but this imbalance does not affect Modern Masters because this set will stop at Alara Reborn.
If I randomly want to pull Rare and Mythic Rare Modern staples, the sets I am interested in pulling from are the Core Sets, Mirrodin, Ravnica, Zendikar, Worldwake, Scars of Mirrodin, New Phyrexia, and Innistrad. Of course, its a lot cheaper to buy these cards one by one off eBay!!
