I saw your post somewhere else, too. Wondering what you mean by "true" common ancestors. :) Thinking you might be indicating just how many times over you are related and through how many lines rather than just 'true' ancestors. Another words, you may have MULTIPLE common ancestors while the 'hint' may only lead back to one set of common ancestors.
I too have many colonial lines. I've had a few matches that show up as a match to this line or that line, but in looking more closely, I realize I'm likely related to that tree holder several times over. I remember one fellow family genealogist that said, I'm realizing how many times I'm related to myself.
:) Take me back to the 1600s and 1700s and I've discovered my paternal grandmother's ancestors signed deeds that my maternal grandmother's ancestors witnessed. Same grandmothers, different ancestors worshipped at the same church. Absolutely amazing.