Relationship between pH and volume
If correctly prepared, the pH should have been the same. Logically, consider if the pH would be different if measured in 5 separate 100 ml samples poured from the 500 ml preparation.

Why might the pH differ in your tests? Some possibilities:


One container was not completely rinsed after cleaning and had some (often alkaline) detergent clinging to it.
The preparation absorbs $\ce{CO2}$ from the air, so the layer on top is more acidic.
Error in measurement.


Unless you have a buffered system, pH may vary abruptly with a slight change in ingredients.