When the engineers are designing a lens, the 70-300 is a target focal range they design to, but it's not important that they hit it exactly as long as it covers the advertised range. As they tweak the lens characteristics to get a suitably sharp and quality image for the target cost, the actual focal lengths may change slightly. Eventually, when everything is "good enough" it's time to mass produce it, as it's not worth the time to have the staff readjust the design to stop right at 300.

It's not at all uncommon to have lenses, even prime lenses, have an actual measured focal length to be slightly different than the listed focal length. Nobody is really going to care if their 50mm prime lens is actually 48.9mm or 52.1mm, but it's close enough to compare it to other "50mm" primes, and searching for a lens in a catalog where it listed 48.9 instead of nice round numbers like 50 would be even more obnoxious than lens shopping already is. When you are looking for a 35mm lens, you are really looking at a class of lenses where the focal length is 'close enough' to 35mm.

Then, adding to all this, the focal length of a lens can often change with the focus distance, and for photography lenses this is almost always the case. This is known as "focus breathing", and only for cinema lenses does anyone bother to design the lens to maintain the same focal length throughout the focus distance range. Supposedly the focal length is supposed to be specified with the lens focused at infinity, but I don't know if that's true.