Charlotte, there are many reasons the bedrooms might be off, not the least of which is changing regulations over the years. On an older house, rooms that were once not considered bedrooms, may now be exactly that, as the regulations around what is a "bedroom" have loosened. So perhaps the house was once a 4 bedroom, but under current regulations can now be called 5 or 6 bedroom. What is really important is really the amount of people living in the home. If there are 5 or even 6 people living in a home, with a mix of adults and children, a "4-bedroom" septic should handle that no problem.
But also, as someone mentioned, private septic systems can be made to handle virtually any number of bedrooms these days. I had a listing once for a $3 million, 10,000 sq ft, 7 bedroom home, that had septic and a well. So anything can be done!
I actually live in Tyngsboro myself, so if I can be of any help don't hesitate to contact me anytime!