28138digger, I too have seen this quite often, where those markers aren't accurate as to location. The reason is simple I believe: They place them where is easy pull-off for lookie-lous to read the plaque. Ie.: the closest spot to make an auto-pull-out shoulder, or wherever. And it's "close enough", so "what's the worry?" So for example: there was a battle in CA war with Mexico, that was fought near where I live. And there is a historical plaque marking the "spot", with a little blurb. However, no one really knows where the battle was litteraly fought (as it was just a 1 day event back in the 1840s), other than knowing the general area of what is today cow pastures, row crops, etc... in a rural area. So when someone put the plaque there years ago, they just chose a spot where it would be safe for persons to pull off, on the country road, and read such a thing.
Another plaque I can think of says something to the effect of "3 miles west of here was the site of such & such stage stop, that operated from such & such date, to such & such date, blah blah blah". But the actual location (which we found from our own research) is miles from this coordinate. The reason for the errant marker, is that in the 1960s, when freeways were updated to include on & off ramps, the marker was moved up the road a few miles, so that it was convenient to the next freeway off-ramp. So now instead of being "3 miles west", it is more like 5 miles SW from the current location of the marker. Not worthy to have to re-make an expensive brass plaque. But the humourous thing is, I'm sure people reading that, since the mid 1960s, have no doubt trekked off hiking litterally 3 miles to the west of the plaque, and will never find the site
For these, and other reasons, the plaques are often geographically off for our purposes, but "close enough" for tourist's sakes.
It is the plaque for the shot heard around the world. Right in the middle of a busy street in downtown Boston. If you ever visit the site, read fast and get on the sidewalk asap 
