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.

{
    std::vector<std::vector<unsigned char> > vSolutions;
    if (!Solver(scriptPubKey, whichType, vSolutions))
        return false;
    if (whichType == TX_MULTISIG)
    {
        unsigned char m = vSolutions.front()[0];
        unsigned char n = vSolutions.back()[0];
        
        if (n < 1 || n > 3)
            return false;
        if (m < 1 || m > n)
            return false;
    } else if (whichType == TX_NULL_DATA &&
               (!fAcceptDatacarrier || scriptPubKey.size() > nMaxDatacarrierBytes))
          return false;
    return whichType != TX_NONSTANDARD;
}
bool IsStandardTx(const CTransaction& tx, std::string& reason)