There are at least two issues involved here:

• Focal lengths, particularly with zoom lenses, are approximated to the nearest "standard" number. You'll almost never see a lens marked as a 242mm or 256mm lens, even if that is their actual focal length. They'll both be sold as 250mm lenses. In the case of a 70-300mm lens, the mark on the barrel for 250mm may not be in the precise spot it should be, either.
• Focal lengths are measured when a lens is focused at infinity. That is, the focal length is measured based on how far collimated light striking the front of the lens will be focused behind the lens. As lenses are focused on distances closer than infinity, their angles of view (AoV) often change.

Two lenses that have the same FoV at 200mm focused on far objects may have very different FoVs when focused at closer distances. The AF-S Nikkor 70-200mm f/2.8G VR II, for example, was notorious for giving a roughly 145mm AoV when set to 200mm and focused at the lens' minimum focus distance (MFD). In contrast, the competing Canon EF 70-200mm f/2.8 L IS II gave an approximately 195mm AoV at a similar MFD. The newer AF-S Nikkor 70-200mm f/2.8E FL VR also gives an AoV much closer to what one would expect from a 200mm lens, even when focused at shorter distances.