Officials from the Department of Education said the degree of pressure on those involved in establishing the calculated grades system was "extreme".
Dalton Tattan, assistant secretary at the Department of Education, told the Oireachtas Education Committee that the grades coding error was "unfortunate".
The errors affected 6,500 Leaving Cert students, 6,100 of whom were awarded higher marks.
More than 400 students who missed out on college courses due to the calculated grades error were offered their places.
Mr Tattan said the timetable set out to create the system for Leaving Cert grades was "extremely ambitious".
Work is under way to prepare for next year's Leaving Certs exams, which officials at the department said they are "determined" to run.
Mr Tattan said that while there were coding errors, the degree of pressure on those creating the system was "extreme".
Around 2,800 students will sit Leaving Cert exams across 600 schools next Monday.
"The Leaving Certificate class of 2020 faced a unique set of circumstances and the decision to provide students with calculated grades was taken with the very best interests of students at heart and in full consultation with the partners in education," Mr Tattan said.
"Some initial scoping of the review has taken place and legal advice is currently being sought, having regard to certain litigation in being started."
Senator Fiona O'Loughlin queried whether the system was tested before it went live.
Mr Tattan said it was shocking to find out about the errors, which he said happened in the same section of coding.
He added that the degree of pressure on those creating the system was "extreme".
Mr Tattan said: "The (Education) minister (Norma Foley) has already apologised and I would like to reiterate that apology.
"All (errors) related to one small section of code described as a family of the coding. It was a particularly complex code."
The errors have been attributed to mistakes in the coding programme made by Polymetrika, the company hired to oversee the calculated grades system at a cost of 193,000 euro.
Harold Hislop, chief inspector in the Department of Education, said it was impossible to run checks.
Mr Hislop told the committee that the department tried to get contingency cover but it was "impossible" to get staff.
"Each of the agencies that would have provided that expertise were taken up by their own processes which was hoovering up the international expertise," he added.
Cork Sinn Fein TD Donnchadh O Laoghaire said he "couldn't fathom" how the errors occurred.
He said that around 8,000 higher grades awarded last year have led to a "ripple effect" on other students.
The TD queried whether it would be possible to provide deferred offers to students who were pushed out of their college course by the higher grades.
Mr Tattan said the department reviewed that option but said it was not possible to do without rerunning the entire process.
"I don't think it's possible to do that without doing a full rerun. We did explore it," he added.
Mr Tattan also said he didn't believe any students were treated unfairly, adding that the department "went out of their way" to ensure the system was fair.
There are currently 20 separate legal proceedings under way challenging the Leaving Cert calculated grade system.