Why is there a range of prices for the same beverage at different dispensers?

Stomper Vanillaijs recently noticed a disparity between the prices of an orange juice from different dispensers located around the island. 

According to Vanillaijs, the orange juice dispensers in Orchard and Selegie charge only $2 for a cup but a smaller cup from dispensers in Sengkang costs $3 instead. 

According to the Stomper, the machines in Sengkang were constantly breaking down in the past, and back when it cost $2 per cup, they would only dispense about half a cup of orange juice.

At some point, the old machines in Sengkang were replaced and prices of the beverage also increased to $3, despite the smaller cup. 

Said the disgruntled customer:

“I think a 50% increase in prices is too much, because it is only $2 even in an area like Orchard.”