in v0.12 https://github.com/bitcoin/bitcoin/pull/7081 (by @luke-jr) was merged adding `-bytespersigop` (with default of 20).

this is preventing simple bare multisig transactions from being accepted into mempool when the size is < 400 bytes.  
eg; https://gist.github.com/rubensayshi/ac4f617207f7a50559e85d61c05800be

from 1-of-7 and onwards they are accepted because the ratio of bytes:sigops is high enough.  
also having 2 or more inputs will work because it again tips the ratio of bytes:sigops over.

the reason mainly being that `GetLegacySigOpCount` calls `GetSigOpCount` with `fAccurate=false`.

It looks like changing `GetLegacySigOpCount` to use `fAccurate=true` would cause side effects in the few other places (`ConnectBlock`, `CheckBlock`, `CreateNewBlock`) and simply adding `fAccurate` as arg to `GetLegacySigOpCount` would affect the value stored in `CTxMemPoolEntry.sigOpCount` which in turn affects a bunch of other things.

}
    
    if (IsWitnessEnabled(pindex->pprev, chainparams.GetConsensus())) {
        flags |= SCRIPT_VERIFY_WITNESS;
    }
    int64_t nTime2 = GetTimeMicros(); nTimeForks += nTime2 - nTime1;
    LogPrint("bench", "    - Fork checks: %.2fms [%.2fs]\n", 0.001 * (nTime2 - nTime1), nTimeForks * 0.000001);
    CBlockUndo blockundo;
    CCheckQueueControl<CScriptCheck> control(fScriptChecks && nScriptCheckThreads ? &scriptcheckqueue : NULL);
    std::vector<uint256> vOrphanErase;
    std::vector<int> prevheights;
    CAmount nFees = 0;
    int nInputs = 0;
    int64_t nSigOpsCost = 0;
    CDiskTxPos pos(pindex->GetBlockPos(), GetSizeOfCompactSize(block.vtx.size()));
    std::vector<std::pair<uint256, CDiskTxPos> > vPos;
    vPos.reserve(block.vtx.size());
    blockundo.vtxundo.reserve(block.vtx.size() - 1);
    for (unsigned int i = 0; i < block.vtx.size(); i++)
    {
        const CTransaction &tx = block.vtx[i];
        nInputs += tx.vin.size();
        if (!tx.IsCoinBase())
        {
            if (!view.HaveInputs(tx))
                return state.DoS(100, error("ConnectBlock(): inputs missing/spent"),
                                 REJECT_INVALID, "bad-txns-inputs-missingorspent");