There are two things that have gotten confused since the grace period came about 6-7 years ago.
Prior to that time there was a "run out" period. It was a time that allowed participants to submit reimbursement requests for expenses incurred during the earlier plan year. Sometimes it was only 30 days, but could be as long as 75 days. So for a 2012 calendar year plan, participants could submit claims until March 15, 2013. The claims had to be for expenses incurred during 2012.
Then the grace period was introduced. Unfortunately the time frames for both items were often exactly the same. It was possible to add an additional time frame for an additional run out period after the grace period, but no adminstrator wanted to keep the books open that long.
So a grace period (and this DOES have to be in the plan document) allows a participant to make an election for 2012 and use that money for expenses incurred from January 1, 2012 until March 15, 2013.
So, the key issue is often making sure an expense in January 2013 is reimbursed from the proper account. It SHOULD be paid from any 2012 money remaining, but sometimes the accounting isn't done correctly.
This is very possibly what has happened. I'm sure it is also some confusion between grace period and run out period. Hopefully, this gives you enough to go back and work through the plan years and reach the correct amount.
Under no circumstances should you have amounts still sitting in prior years accounts. It should be forfeited and retained by the employer. But make sure you've accounted correctly.