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.

*
 * See consensus/consensus.h for flag definitions.
 */
bool CheckFinalTx(const CTransaction &tx, int flags = -1);
/**
 * Test whether the LockPoints height and time are still valid on the current chain
 */
bool TestLockPointValidity(const LockPoints* lp);
/**
 * Check if transaction is final per BIP 68 sequence numbers and can be included in a block.
 * Consensus critical. Takes as input a list of heights at which tx's inputs (in order) confirmed.
 */
bool SequenceLocks(const CTransaction &tx, int flags, std::vector<int>* prevHeights, const CBlockIndex& block);
/**
 * Check if transaction will be BIP 68 final in the next block to be created.
 *
 * Simulates calling SequenceLocks() with data from the tip of the current active chain.
 * Optionally stores in LockPoints the resulting height and time calculated and the hash
 * of the block needed for calculation or skips the calculation and uses the LockPoints
 * passed in for evaluation.
 * The LockPoints should not be considered valid if CheckSequenceLocks returns false.
 *
 * See consensus/consensus.h for flag definitions.
 */
bool CheckSequenceLocks(const CTransaction &tx, int flags, LockPoints* lp = NULL, bool useExistingLockPoints = false);
/**
 * Closure representing one script verification
 * Note that this stores references to the spending transaction 
 */
class CScriptCheck