According to parent company Liberty Global, Virgin Media originally said it had connected 465,000 new premises in the UK and Ireland in 2016.
But following a review, that number has been revised down to 314,000.
Liberty said Virgin had exaggerated how fast it could complete the connections.
In a regulatory filing, the US company explained that - under Virgin's own reporting standards - the figures included premises where installation work was "essentially complete" but not actually finished.
That included 142,000 sites that had been expected to be connected during the first three months of 2017.
However, Liberty said that in late February 2017, it had discovered that "the construction work necessary to connect a substantial number of [these premises] had not progressed as originally understood".
This prompted it to carry out a review, in which it found "the completion status" of some premises had been "misrepresented".
Liberty also said Virgin Media had "inadvertently" classified a further 9,000 unfinished premises as connected, when they had not been.
Dan Howdle, a consumer telecoms analyst at Cable.co.uk, told the BBC: "Clearly the way in which the numbers had been reported failed to preclude the possibility of jumping the gun, though there is no evidence an intent to mislead lay behind it."
He added: "Virgin Media appears to be dealing with the issue decisively, though mistakes resulting in stories of this nature are unlikely to leave a brand completely unscathed."
Four Virgin Media employees have been suspended in connection with Liberty's investigation and could face disciplinary action "including dismissal".
The company said the setback would affect the total number of premises Virgin Media connected during the first half of 2017.
However, it said it had taken steps to speed up Project Lightning, including appointing a new managing director, Robert Evans, to lead the scheme.
It also said Virgin would no longer include uncompleted connections in its figures.
Launched in 2015, Virgin's Project Lightning is designed to connect about four million more UK homes and businesses to broadband speeds of 300Mbs.
By the end of December it had reached 567,000 premises in total - down from the 718,000 previously reported.

The Harrogate Autumn Flower Show will use specialist technology to ensure the winning fruit is of the Gigantomo variety.
The giant entries must be from that strain of plant to win the Â£1,000 prize.
It comes after competitors raised concerns unscrupulous growers might sneak in a rogue type of tomato.
Show director Nick Smith said they wanted to be certain the winner "had the right pedigree."
"Giant veg growing is great fun and tremendously popular with our visitors, but it also has a serious side, especially with such a big prize at stake," he added.
The Gigantomo class, developed for its huge red fruits, was launched in January, with the final weigh-in on Friday.
Plant specialist and prize sponsors Van Meuwen said it is the product of almost two decades work by breeders in the UK and United States.
Dutch specialists will test the plants to make sure the entries are what they claim to be and the winner could scoop a further Â£5,000 if the heaviest fruit sets a new world record.
The Guinness record for the heaviest tomato stands at 7lb 12oz (3.5kg).

The woman was picked up from Candlemaker Row, Edinburgh, near the Greyfriars Bobby's statue, at 03:30am.
She was taken to a layby just past the Hillend Ski Slope, near the Boghall Farm in Midlothian where she was subjected to a serious sexual assault..
She was then left near the Sheriffhall roundabout on the city bypass.
Police Scotland are appealing for any witnesses who may have seen the victim or the car.
The suspect was described as being of South Asian appearance, aged 35-40,  about 5ft 5in - 5ft 6in, of slim build.
He had dark, cropped hair and was clean-shaven. He was wearing dark clothing and spoke with an Edinburgh accent.
The car was described as being a five-door saloon, which was possibly a navy colour.
Det Sgt Keith Mackay said:  "This was a frightening experience for the woman, who is now in the care of her family.
"We are eager to trace this man and are appealing for witnesses.  Anyone who recognises the man's description, or the description of the car, is urged to come forward."
Virgin Media has suspended four members of staff and begun an investigation after it admitted overstating the expansion of its superfast broadband network, dubbed Project Lightning.
[NEXT_CONCEPT]
Gardeners trying to cheat their way to the top tomato prize at a horticultural show will be weeded out with DNA tests.
[NEXT_CONCEPT]
A 31-year-old woman has been sexually assaulted after getting into a car she believed was a private hire taxi.