Gap between different kinds of gas costs

[Arisawa-v]

hello I’m measuring gas with debug_traceCall. My contract returns abi.encode(gasleft()).

With a very high gas limit I get:

Gross Gas Cost (start − final gasleft()): ~2.46M

Net Gas Cost (trace.gasUsed, maybe due to refunds): ~2.28M

But the tx only succeeds with at least 2.51M gas; setting the limit to ~gross OOGs. Calldata is short.

Is the gap between gross and non-OOG gas due to the EIP-150 63/64 rule? Is the gap between gross and net gas due to refunds? How do I correctly compute the minimal non-OOG gas?