Vyper OPCODES gas estimate at compile time

Question

I am trying to dissect Vyper at lower levels to understand how the gas estimates are generated. I tried following the yellow paper and I searched every single line of the compiler but I can't get it right... Is there anyone that can lead me in the right direction? Giving me a hint about how it's done and why the git compiler gives out much higher estimates than the online one? Thanks to anyone who will provide me something.