I believe he's referring to their inability to divide 10/7 to calculate how much they're actually paying for each dragon egg, implying that he believes that if players knew each egg was almost a dollar and a half then they wouldn't purchase the eggs.
It is (explicitly, as a planned-and-tested-for-outcome) to cause players to not automatically attach "$3.50" to an in-game purchase which costs "3 dragon egs", thus not activating as much resistance to purchasing as it would if the prices were trivially calculable. (For instance, if you sold 10 dragon eggs for $10.)
You want to make it very obvious to people what they're paying for eggs, but very, very, very easy to spend eggs once they have them, both because a) it's in your best interest that they run out of eggs and re-buy them and b) because you will not be able to recognize revenues for eggs which are bought but not converted into in-game assets. (Revenue recognition for this is a deep and weird topic. Ask me some other time.)
There may also be a "hot dogs in packages of 8, buns in packages of 6" dimension to choosing the odd-sized in-game prices/currency-bundles. A user who has a little in-game change left may feel they have to add more, to get full value on their previous purchases.
