300 mm is always the same, but there are some factors that have to be accounted for. One factor that you would have to account for is that that 300 mm starts at the front of the lens, not at the tripod mount or body. Most lense designs "fold" the light paths to shorten the lense. That means your 300 mm lense is not actually 300 mm long in size. Your 28 - 300 is most likely your physically shortest lense while the 55 - 300 and 150 - 600 are physically much longer. If you set your camera body at 12' and just changed out lenses, and there is a 6" difference where the front of the lense element is, there is already a greater than 4% error that has been introduce into the focal length. That and the that manufacturing range mentioned by Apaflo is more than enough to produce the differences you have noticed. Try setting the front of each lense at 12' (move the tripod), set the lenses at 300 mm, and see if that seems to change things back to a more equal "300 mm" setting. Report back with pictures if you can.

But that refers focal length defined as the distance of the lens from the sensor when focused at infinity.

When not focused at infinity, focal length is actually an "effective focal length". And how that is affected by focus distance depends on the lens design. Up until 25 years or so ago lenses focused closer by moving the entire lens away from the sensor. But now almost all lenses use what is called an Internal Focus design, which has a fixed distance to the sensor and instead moves internal groups of elements in relation to each other.

One effect, most noticed when neither the front or back elements of the lens moves, is that the effective focal length becomes much shorter as the minimum focus distance is approached.

Which is to say, your 300mm lens is close to 300mm when focused on the moon, but it might be only 200mm when focused as close as it can. Or 220, or 250mm... each might be very different! But that is what allows a 12 foot minimum focus distance rather than only 20 feet.

The significance is that there is nothing wrong with any of your lenses in the sense that at one set focus distance they all exhibit different effective focal lengths. It is very much to be expected.