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.

SCRIPT_VERIFY_DERSIG |
                                                         SCRIPT_VERIFY_STRICTENC |
                                                         SCRIPT_VERIFY_MINIMALDATA |
                                                         SCRIPT_VERIFY_NULLDUMMY |
                                                         SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
                                                         SCRIPT_VERIFY_CLEANSTACK |
                                                         SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
                                                         SCRIPT_VERIFY_CHECKSEQUENCEVERIFY |
                                                         SCRIPT_VERIFY_LOW_S |
                                                         SCRIPT_VERIFY_WITNESS |
                                                         SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_WITNESS_PROGRAM;
static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;
static const unsigned int STANDARD_LOCKTIME_VERIFY_FLAGS = LOCKTIME_VERIFY_SEQUENCE |
                                                           LOCKTIME_MEDIAN_TIME_PAST;
bool IsStandard(const CScript& scriptPubKey, txnouttype& whichType);
    /**
     * Check for standard transaction types
     * @return True if all outputs (scriptPubKeys) use only standard transaction forms
     */
bool IsStandardTx(const CTransaction& tx, std::string& reason);
    /**
     * Check for standard transaction types
     * @param[in] mapInputs    Map of previous transactions that have outputs we're spending
     * @return True if all inputs (scriptSigs) use only standard transaction forms
     */
bool AreInputsStandard(const CTransaction& tx, const CCoinsViewCache& mapInputs);
int64_t GetVirtualTransactionSize(int64_t nWeight);
int64_t GetVirtualTransactionSize(const CTransaction& tx);
#endif