The result of last month’s London Mayoral election on 5 May was delayed by several hours after staff had to manually query a bug-stricken database.
Sadiq Khan was elected, but the result was not declared until the Saturday.
One of the firms behind the software, DRS, revealed new details of the mishap at the London Assembly on Thursday.
Staff had to collate discrepancies in a spreadsheet, rather than automatically, when tallying final results, said chief executive Steve Gowers.
It was during the late afternoon the day after the election that “discrepancies” in different sets of results were noticed.
The system was provided by IntElect, a joint project between DRS Data Services and ERS Group.
A resulting investigation found that the underlying data, the actual numbers of votes in the database, was correct – but the counting software was not reporting it accurately.
The decision was then taken to disregard the system’s ability to collate results from each constituency – querying this data was instead done by programmers.
“The manual part of the process meant that we took the output of those queries and put them into a spreadsheet to add them up,” explained Mr Gowers.
Another meeting to discuss what happened will not be held until next month, a spokeswoman for London Elects told the BBC.
“There’ll be a further meeting and further investigations, then a report will be released with recommendations,” she said.
Electoral law expert Prof Bob Watt at Buckingham University said he was concerned about the use of electronic counting systems.
“The bigger issue is of course just how stable these machines are and that’s something that I have a great deal of worry about and have had for some time,” he told the BBC.
Electronic voting machines are not currently used by UK voters when casting ballots, but counting software is used in the London Mayoral and Assembly elections.
In this case, the Assembly election was not affected by the glitch, according to City Hall.
The London Elects spokeswoman said that without electronic aid it would take “three days” to count the votes from various ballots.
Electronic counting systems will not be used to count votes in the EU referendum on 23 June.