Gas, gas, and more gas.

For the most part, I've learned to accept that gas prices will always fluctuate. I get that. I understand that it is dependent on factors like the world market. However, what I don't get is why prices aren't uniform. Take for example the other night. I was running low on gas so I swung by a Shell station. I was surprised at the price! It was 38.27 per liter! I was shocked because earlier that morning, I saw that it only was 37.22. That's over 1 peso difference! And yes, both were shell stations. What's up with that?