Variation in Reproduction

You must have noticed a general trend in living beings :

Each generation of a living being is followed by the next generation belonging to the same species.

• An apple seed if sown results in the growth of an apple tree only. 
• Only a chick hatches out of an egg laid by a hen
• A human being gives birth to another human being only

But if you take a closer look, you will see that;

• Some organisms do not look different from one another
• For example - In a sunflower garden, all sunflowers look very similar to each other and we find very little variation among the individual plants

• Some possess certain basic characteristics but organisms evidently vary from one another
• For example - Human beings generally have the same physiology: 2 hands, 2 legs, 2 eyes, 1 nose etc. However different human beings look evidently different from one another.

Why is this so?