WASHINGTON —  Only half as many muni advisory firms are registered with the Municipal Securities Rulemaking Board compared to those registered with the Securities and Exchange Commission, even though advisory firms are now required to sign up with both regulators under the Dodd-Frank Wall Street Reform and Consumer Protection Act.

As of Monday morning, 878 muni advisers were registered with the SEC but only 442 had signed up with the board. Though the commission began requiring that municipal advisers register with it by Oct. 1, to coincide with the effective date of Dodd-Frank provisions regulating advisers, the MSRB gave advisers until Friday to register with their separate system.

However board officials contend there are explanations for the large discrepancy, which market participants previously attributed to confusion about the existence of dual registration. MSRB executive director Lynnette Hotchkiss said that the list of SEC registrations is not necessarily an indicator of who should register with the board.

“We have been informed that many firms registered with the SEC as a precaution until additional guidance was given,” she said, adding that an adviser would not have to register by the MSRB’s Friday deadline if it did not have active clients.

In addition, Hotchkiss said that the posting of MSRB registrants on its website may be delayed for one to two weeks because of “any number of factors including payment confirmation or follow-up issues.”

“As such, the MSRB list shouldn’t be dispositive of the advisers who have registered by the deadline,” Hotchkiss said.

Though SEC registration is free, MSRB registration requires the payment of an initial fee of $100 as well as an annual fee of $500 per firm.

MSRB spokesman Jennifer Galloway said she could not provide a number of pending registrations because many are processing all the time. However, she said the board received a significant number of registrations last week.

Many advisers, faced with a crush of end-of-year business, said late last month that they had planned to sign up with the board when their workload lessened.

Under Dodd-Frank, “municipal advisers” are defined as businesses that advise municipal entities or conduit borrowers about muni financial products or the issuance of municipal securities.

They include financial advisers, swap advisers, and guaranteed-investment contract brokers.

The law also covers those who solicit certain types of business from municipal entities on behalf of unrelated dealers or advisers.

It exempts from the adviser definition dealers serving as underwriters, attorneys offering legal advice or providing “traditional” legal services, as well as engineers providing engineering advice.

Phone calls Monday to a handful of firms signed up with the SEC but not the MSRB as an adviser confirmed many of the reasons for delays cited by ­Hotchkiss.

Robert Stone, vice president of Androscoggin Savings Bank in Lewiston, Maine, said his bank’s registration with the MSRB is pending until the check it sent to the board on Thursday clears.

Stone said that the bank, which is registered with the SEC as a solicitor, refers municipal borrower clients to a Massachusetts-based advisory firm.

Were it not for that firm, Androscoggin would not have known about the registration requirements.

“This is all new territory for us,” Stone said.