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.

break;
                    }
                }
                uiInterface.InitMessage(_("Verifying blocks..."));
                if (fHavePruned && GetArg("-checkblocks", DEFAULT_CHECKBLOCKS) > MIN_BLOCKS_TO_KEEP) {
                    LogPrintf("Prune: pruned datadir may not have more than %d blocks; only checking available blocks",
                        MIN_BLOCKS_TO_KEEP);
                }
                {
                    LOCK(cs_main);
                    CBlockIndex* tip = chainActive.Tip();
                    if (tip && tip->nTime > GetAdjustedTime() + 2 * 60 * 60) {
                        strLoadError = _("The block database contains a block which appears to be from the future. "
                                "This may be due to your computer's date and time being set incorrectly. "
                                "Only rebuild the block database if you are sure that your computer's date and time are correct");
                        break;
                    }
                }
                if (!CVerifyDB().VerifyDB(chainparams, pcoinsdbview, GetArg("-checklevel", DEFAULT_CHECKLEVEL),
                              GetArg("-checkblocks", DEFAULT_CHECKBLOCKS))) {
                    strLoadError = _("Corrupted block database detected");
                    break;
                }
            } catch (const std::exception& e) {
                if (fDebug) LogPrintf("%s\n", e.what());
                strLoadError = _("Error opening block database");
                break;
            }
            fLoaded = true;
        } while(false);