Teachers are intentionally bumping up students' predicted A-level grades to help them win places at top universities, the head of Ucas has suggested.
It comes as institutions are now "more flexible" with grade requirements amid intense competition to attract students, Mary Curnock Cook said.
She said some teachers had told her they are "over-predicting" sixth-formers' results to help them secure initial offers from universities that may be asking for high grades.
Ucas figures show a nine percentage point rise since 2010 in the numbers of students predicted to score at least two A grades and a B at A-level, to 63% in 2015.
However, separate statistics show that students could then be accepted with lower results as many institutions are now accepting more candidates who fail to score their expected grades.
Last year, more than half of students accepted on to degree courses had missed their results by two or more grades, spread over three qualifications, Ms Curnock Cook said.
Speaking at a conference on higher education at Wellington College, Berkshire, she said: "I talk to a lot of schools and people who advise students and in the past I would have said 'surely you wouldn't be over-predicting your students on purpose?', and actually just this last summer really, I had teachers coming back to me and saying 'actually, yes we would'.
"I'll show you why, because actually, accepted applicants, the number who are being accepted with quite significant discounts on their offers and their predicted grades has grown quite a lot.
"52% of A-level accepts have missed their grades by two or more grades over the portfolio of three (A-levels)."
Over-prediction of grades has always occurred, Ms Curnock Cook said, but she indicated that it is becoming more common.
But she added: "Offers are being discounted at confirmation time, and we can see that.
"We can see that because the lifting of the number controls has increased competition, amongst universities to recruit students, you can see that happening.
"For example, of the proportion accepted to higher tariff universities, about 44% of those with BBB in their A-levels got a place at higher tariff institutions, compared to just 20% in 2011."
As universities compete for students they are being "more flexible with their entry grades," she added.
Speaking after, Ms Curnock Cook said: "It's not like it's come out of nowhere, predicted grades have always been slightly inaccurate and they've always been more likely to be over-predicted.
"I think people have thought of predicted grades as a way of signalling what they think a person is capable of, if everything goes perfectly. So it's not a huge change, but the way that the market has played out means that some teachers will take a strategic approach to their predictions."
Professor Michael Arthur, provost and president of University College London, said that his institution's standard offer is one A* grade and two As or three A grades, adding that last year, "we did drop down one grade for about 9% or so of students that we admitted".