estimateGas confusions

[novaknole]

I'm trying to understand how estimateGas works in current geth. This question is not related to ethers, but I'd appreciate your experienced eye to take a quick look at this. Thank you.

I have the following contract:

contract Second {
    uint public k;

    function callSecond() public {
        for(uint i=0; i<10;i++) k++;

        require(5==4, "fail");
    }

}

contract Main {
    uint public z = 10;

    function callMain(Second addr, uint o) public {
        for(uint i=0; i<10;i++) z++;

        z = o;

        address(addr).call(abi.encodeWithSignature("callSecond()"));
    }
}

If I run the callMain transaction directly(without estimation or whatsover), it says that gasUsed is 55834. Though, estimateGas from ethers returns 58955.

I wonder why estimateGas didn't return smaller value closer to 55834. The reason I am expecting this is because of binary search. Geth uses binary search and tries different values and finds the smallest gas limit for which tx succeeds, but it's interesting why for example it didn't return 57000 instead of 58955. Then I looked into the implementation

In my case, lo = 55834 - 1 = 55833, optimisticGas = (55833 + 0 + 2300) * 64/63 = 59055 and since optimisticGas < hi, then hi = optimisticGas = 59055 according to the code. This all suggests that binary search here happens between 55833 and 50102 and no chance it would end up with 58955 (see the binary search). For simplicity, I wrote the following in js:

let lo = 55833;
let hi = 59055;

while (lo + 1 < hi) {
   let mid = Math.round((hi + lo) / 2);
   if( (hi-lo)/hi < 0.015) {
	break
    }
    
    if (mid > lo*2) {
	    mid = Math.round(lo * 2)
    }
    try {
	 await contract.callStatic.callMain(secondAddress, num, {gasLimit:mid});
	 hi = mid;
    }catch{
	 lo = mid;
    }
}
console.log(hi)

It logs 56639 instead of 58955.

Any idea ? Thanks in advance for your advices.