The upper end of that range applies to the species. 'Burkii' is a cultivar selected for its very upright habit and smaller pyramidal stature. 25' is usually listed as a "mature" height but it could get as much as 40' over time. Conifers just don't magically stop growing once they are mature - growth will continue until the tree dies.

Thanks - If it keeps growing, what is the point of people listing a "mature height?" What is the logic behind that? Is it that the growth rate slows? Also, do all trees keep growing (example - Nellie Stevens Holly)?

Yes, all trees keep growing until they die :-) Mature height is never a set figure - it is a range that is determined in part by location and in part by growing conditions - optimum location and growing conditions will result in sizes at the upper end of the range.

Trees are genetically programmed to a general size. Once they are fully mature and have achieved close to their genetic potential, growth slows significantly. Never fully stops but, depending on species, may be so slow as to be virtually unobservable.

For example, many so-called dwarf types of trees are only considered dwarf by virtue of their rate of growth.......dwarf conifers in particular. If they live long enough many dwarf conifers can reach impressive sizes well exceeding what is published as a normal "mature" size.

And yes, this applies to Nellie as well :-) Although generally listed as "maturing" between 20-30' (depending on what source you read), there are older, well-established stands of this holly in parts of the South that exceed 40'.