A straight run is a 1:1 ratio of males and females, or an equal amount of each. A "pullet" is a female.
The 1:1 is an 'average'. The actual ratio varies. When purchasing chicks the true count comes out of what ever hatches. Eggs are hatched 10,000 to 100,000 at a time. If there are more females to males in that hatch you get more females..... You get a straight run by them pulling your 25, 50 or 100 from those. No one actually sexes that many birds without an order for all males or all females.