I read over exertion can lead to slight elevation of platelet due to dehydration. how come fasting for 12 hours doesn't affect platelet count result ?

Depends : Traditionally, fasting for bloodwork does not require you to not drink water. In fact, you're encouraged to drink water in order to maintain normal hydration while you are restricting other electrolytes and sugar which could cause aberrant values. Technically the dehydration cause by refraining from liquids may potentially be sufficient to falsely elevate both the hematocrit and the platelet count.

Release from organs: Exercise causes release of platelets from internal organs, especially spleen. Dehydration affects all cell types, not just platelets.