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.

indexed_transaction_set::const_iterator it2 = mapTx.find(txin.prevout.hash);
            if (it2 != mapTx.end()) {
                const CTransaction& tx2 = it2->GetTx();
                assert(tx2.vout.size() > txin.prevout.n && !tx2.vout[txin.prevout.n].IsNull());
                fDependsWait = true;
                if (setParentCheck.insert(it2).second) {
                    parentSizes += it2->GetTxSize();
                    parentSigOpCost += it2->GetSigOpCost();
                }
            } else {
                const CCoins* coins = pcoins->AccessCoins(txin.prevout.hash);
                assert(coins && coins->IsAvailable(txin.prevout.n));
            }
            
            auto it3 = mapNextTx.find(txin.prevout);
            assert(it3 != mapNextTx.end());
            assert(it3->first == &txin.prevout);
            assert(it3->second == &tx);
            i++;
        }
        assert(setParentCheck == GetMemPoolParents(it));
        
        setEntries setAncestors;
        uint64_t nNoLimit = std::numeric_limits<uint64_t>::max();
        std::string dummy;
        CalculateMemPoolAncestors(*it, setAncestors, nNoLimit, nNoLimit, nNoLimit, nNoLimit, dummy);
        uint64_t nCountCheck = setAncestors.size() + 1;
        uint64_t nSizeCheck = it->GetTxSize();