Because you have only a 25% chance of getting one. Flip a coin 10 times and you should get 5 heads, 5 tails....if you get 7:3 then 8:2 then 6:4, does this change the odds? No. You have to remember that the 25% is just the odds...when Mendel (Father of genetics) did his pea plant crosses, he looked at thousands of results to come to the conclusion that you have a 25% chance of pure recessive from two heterozygous individuals....
So, after you average thousands of clutches, I bet you will see the numbers pan out pretty close to a 1:2:1 ratio (Normal:rusty:leucistic).

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