In the mid 80's a developrat offered me £50.000 for 1/2 of my garden which would have given him 1/4 acre of nicely situated building plot. He just could not understand that it was where the Kids played and I didn't want a house next door.
From what I seen it's entirely possible they would have told you it was for one house and then you find it was for 3-4.
We have some friends locally whose parents sold a small strip of their garden to help a "lovely couple" who wanted to build a house and and needed access. They sold it for pennies because they were helping someone out. The purchasers went on build two properties and never came near the place! They had made it seem like they really wanted to move the to area etc,etc.