It also has to do with the certification of the engine.
The PT6 line for instance has been constantly improved/upgraded over time, when the engines originally came out the Ng limit was 100%. Over time, due to the improvments and upgrades many modern variants can easily go ver the 100% Ng limit, however setting their new max as 100% would mean getting the engine recertified (as a new engine rather than a variant), if Pratt and Whitney simply say the newer variant is capable of 106% then no new engine certification is required.