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.

{
    *this = other;
}
double
CTxMemPoolEntry::GetPriority(unsigned int currentHeight) const
{
    double deltaPriority = ((double)(currentHeight-entryHeight)*inChainInputValue)/nModSize;
    double dResult = entryPriority + deltaPriority;
    if (dResult < 0) 
        dResult = 0;
    return dResult;
}
void CTxMemPoolEntry::UpdateFeeDelta(int64_t newFeeDelta)
{
    nModFeesWithDescendants += newFeeDelta - feeDelta;
    nModFeesWithAncestors += newFeeDelta - feeDelta;
    feeDelta = newFeeDelta;
}
void CTxMemPoolEntry::UpdateLockPoints(const LockPoints& lp)
{
    lockPoints = lp;
}
size_t CTxMemPoolEntry::GetTxSize() const
{
    return GetVirtualTransactionSize(nTxWeight);
}
void CTxMemPool::UpdateForDescendants(txiter updateIt, cacheMap &cachedDescendants, const std::set<uint256> &setExclude)