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.
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.