I take it that's when people don't take the time to compare and measure, if possible, some object it stood next to.
Fortunately on some sightings they walk directly next to a tree or just under a branch which can be measured later. I read a report of one ducking under a tree branch that when measured later was nine feet off of the ground. John Green and another researcher when to bluff creek not long after Patterson filmed Patty and they measured where it had walked in front of a tree which I think was 7'6". I don't believe that accounted for the fact that sasquatches keep their knees bent when they walk. Bill Munns later tried to figure out the height of Patty, going to Bluff Creek and tried to guess at the distance and type of camera used and came up with 6'6", which skeptics gleefully repeated claiming Patty wasn't as tall as people had claimed, but I think I'll go with the original measurements which are more accurate.
One I saw stood next to a steep embankment, and the next day I went and stood where it had stood to see how much taller it was compared to me next to the embankment where it stood and I initially came up with nine feet, which I couldn't accept, and when telling people how tall it was I revised it down to a more conservative height because I was already afraid of not being believed.