* Scaling Adjustments: The scaling used to convert raw scores (number of correct answers) to scaled scores (1-36) has been adjusted over time. This is done to maintain score comparability across different test forms, as the difficulty of each ACT test form varies slightly. These adjustments aren't usually publicized in detail.
* Section Weighting: While the individual section scores remain on a 1-36 scale, the method of calculating the composite score might have seen minor adjustments in how the individual section scores are weighted to produce the composite. The exact details are not always publicly available.
* Test Content Changes: The content of the ACT has evolved to reflect changes in high school curricula. This isn't a change to the scoring itself, but it means that comparing scores from the 1980s directly to scores today isn't a perfectly apples-to-apples comparison due to differences in the subjects tested and the level of knowledge assumed.
* Addition of the Writing Section: In 2005, the ACT added a writing section, which is scored separately from the core four tests (English, Math, Reading, Science) and has its own scoring scale.
In short: The basic 1-36 scale and the concept of a composite score remain constant. However, subtle changes in scaling and test content make direct comparisons of scores across decades imperfect. You cannot simply assume a 28 in the 1980s is exactly equivalent to a 28 today.