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.

mapTx.modify(ancestorIt, update_descendant_state(0, nFeeDelta, 0));
            }
        }
    }
    LogPrintf("PrioritiseTransaction: %s priority += %f, fee += %d\n", strHash, dPriorityDelta, FormatMoney(nFeeDelta));
}
void CTxMemPool::ApplyDeltas(const uint256 hash, double &dPriorityDelta, CAmount &nFeeDelta) const
{
    LOCK(cs);
    std::map<uint256, std::pair<double, CAmount> >::const_iterator pos = mapDeltas.find(hash);
    if (pos == mapDeltas.end())
        return;
    const std::pair<double, CAmount> &deltas = pos->second;
    dPriorityDelta += deltas.first;
    nFeeDelta += deltas.second;
}
void CTxMemPool::ClearPrioritisation(const uint256 hash)