Abby Lee Miller will be walking out of federal prison in Victorville, Calif., on May 25 – nearly one month earlier than her originally scheduled release date of June 21. And Star has learned the real reason for the Dance Moms diva’s shortened sentence!

“The reason she is getting released early is because the Bureau of Prisons did not calculate Abby Lee’s good conduct time, or good time credit, when she was sentenced,” a prison insider explained. “It is basically credit given to inmates who maintain good behavior while incarcerated.”

But the 53-year-old won’t be completely free come May, the insider insisted.

“When she is released by from the BOP, that is not the end of her sentence. Once she leaves Victorville, Abby Lee will be on supervised release at home which is also called mandatory parole, for the one to three years.”

“She will have a probation officer who can come to check on her at home or at work or wherever they want at any given time,” the source added.

As previously reported , Miller — who is serving a 366 day prison sentence after being convicted on federal charges that included fraud and money laundering — was supposed to be transferred to a halfway house last month. However, the transfer never materialized due to overcrowding.

According to the insider, the reality star could still move into the house by the end of April.

“There is a really good chance that she will go into a halfway house in April to serve out the remainder of her sentence,” the source revealed. “But that is not a guarantee and they may decide to just keep her at Victorville the entire time, which is rare. The only reason that they would do that is if they really wanted to make an example out of her.”

Miller initially checked into prison in July.