Because future archaeolologists are going to be stupid?
Well, I wouldn't say stupid, but you have to realize that sometimes there's very little information on which to base a guess, and no context. In fact sometimes the closest other thing you can date in the area is from a few thousands of years before or after.
So you find some guy hanged and tossed in a bog, and you really have no idea wth happened there. So you come up with a hypothesis like "we think it was some kind of sacrifice to ensure fertility". And maybe it's right too. Or maybe the guy hanged himself, and they chucked him there instead of burying him because suicide was forbidden. Or whatever.
I really don't think it's necessary to assume anyone stupid to imagine them stumped by something that is a couple of dark ages behind and lacking context, especially if it resembles nothing their own civilization is doing.
So let's say you find a couple of partial skeletons from 5000 years ago and, since those bags aren't really built to withstand roots and geologic kinds pressures, a lot of silicone fluid smeared all over and contaminating the ground around the bones. WTH were those guys doing with fluid silicone? Some kind of ritual anointing the body, maybe?
There's no reason to expect that they'd still use silicon for that (they may have found better ways to enlarge breasts by then) or even that they still are obsessed with gigantic breasts. There are plenty of historically documented societies that, on the contrary, liked small breasts. At least some Roman women for example tied a piece of cloth over their breasts to flatten them and make them look smaller.
You get some people from such a society digging up the silicone-contaminated bones and they might not even imagine anything similar.