Not that it's very important but I'm wondering why doesn't a Canon 18-200 mm lens have visible change in framing between 170 and 200 mm? Why is it declared as a 200 mm lens if it stops zooming at 170-something millimeters?
Or is it something subtle that I'm missing? 30 mm on a lens is usually not negligible so I'd also like to know is it the same throughout the focal range or is it less noticeable at telephoto end?
The focal-length of lenses is measured while focused at infinity. If you were inside a store you likely focused at less than infinity and got a different focal-length than 200mm at the long end. The closer you focus, the more likely you are to be off.
At infinity, there should be a small (probably 7% as @mattdm computed) but noticeable difference. Otherwise the lens was most likely defective.
Are you really seeing no difference, or is it just a slight difference? Take a look at this chart from this tangentally-related answer (thanks to Nick Bedford):
It's dramatic at the beginning, but flattens out at higher focal lengths.
The basic relationship between dimensional field of view and focal length is linear. That means 30mm is a huge difference between, say, 15mm and 45mm, because the linear view is cut to a third. But when you go from 170 to 200, that's just a change of ¹⁷⁰⁄₂₀₀, or 85%. That means 7.5% off each edge, and that's not very much at all.
difference blending mode in Photoshop to compare the photos. Regarding the graph - yeah, basic physics, I guess... What a dope... But how would you then explain the fact that a 70-200 mm lens has a lot smaller (meaning, more zoomed) framed area when used at 200 mm than my lens at 200 mm. I tried the 70-200 mm lens at a camera store just to see the difference between them, though, but it is still quite observable. Why wasn't this lens just named 18-170 mm?
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